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Last Updated: Sunday, 19 June, 2005, 21:31 GMT 22:31 UK
Programme transcript
What follows is a transcript of Panorama's Cannabis:what teenagers need to know. Broadcast Sunday 19 June 2005, 22:15 BST on BBC One

Although taken from a script, this should be checked against transmission for accuracy and to ensure the clear identification of individual speakers.

TITLE MUSIC

CANNABIS SMOKER
I normally only tend to smoke really nice stuff otherwise I just don't bother smoking It's the lightest most creative most inspiring weed, its very sensual...

JUSTIN ROWLATT
These men are cannabis connoisseurs.

CANNABIS SMOKER
What's its like for me to be stoned┐ is much deeper- much deeper than your straight - I've got to deal with stuff consciousness, I've got to pay my bills I've got to go the post office it's... contemplation - I mean I love smoking it right at the end of the day - then I can think about the day and what it means and find meaning in it.. contemplate... absolutely...

JUSTIN ROWLATT
And now the next generation of users are starting younger and smoking more.

CANNABIS SMOKER
I love to wake up and have a spliff, I love to have a spliff before I go to bed. I think it's great.
- We'd even smoke in the playground
- Smoke in the playground on the benches
- Leave school and smoke some more. Come to the park. have some more - we enjoy it so much.

JUSTIN ROWLATT
Over the last two decades, cannabis use among under-sixteen's has increased tenfold*. Tonight we examine new scientific research and ask how much harm is cannabis doing to the mental health of young smokers?

* Source: Schools Health Education Unit

TITLE: CANNABIS: WHAT TEENAGERS NEED TO KNOW

JUSTIN ROWLATT
So when did you guys start smoking?

BOY 2
Started year 7 really.

JUSTIN ROWLATT
So we're talking 11-12 year old kids.

ALL
Yeah.

JUSTIN ROWLATT
And you're telling me that you had your first spliffs, your first joints when you were┐

BOY
We weren't smoking...

JUSTIN ROWLATT
11-12.

BOY3
It was about then, yeah.

BOY2
We weren't smoking it as much at that time.

JUSTIN ROWLATT
That is quite young to start taking any drug really... anything.

BOYS
yes

DRUG WORKER
Can u tell me some slang names for cannabis? Puff, weed, pot, green, skunk, grass.

JUSTIN ROWLATT
A drug education class for 12 year olds. By the age of sixteen 38 per cent of British children will have tried cannabis. Almost one in ten will be a regular user*.

DRUG WORKER
How many of you know people someone that smoke it - we don't want any names┐ right.

JUSTIN ROWLATT
Cannabis has insinuated itself deep into British society.

BOY 2
I don't really class it as a drug.

JUSTIN ROWLATT
You don't class it as a drug?.

BOY 2
No. It's like a fag.

BOY 3
its like a cigarette - its not as much of a big deal to us anymore; its just cannabis. It's just a smoke really.

JUSTIN ROWLATT
Champions of cannabis say the public is ready for full legalisation. And, the popular perception is that the drug is virtually harmless.

PROFESSOR NEIL MCKEGANEY
Parents who used cannabis in the 70's or so developed a view of it as a recreational drug, you know something that doesn't cause anybody any harm, I think in many ways they are conveying that image to their own children now. Now it's penetrated all levels of society and has achieved a kind of unique status of almost not being a drug at all.

JUSTIN ROWLATT
Those attitudes made the decision to lower the legal penalties for adult personal use relatively uncontroversial. But there is now growing evidence that teenage users - for whom possession remains an arrestable offence - may be taking a risk with their long term health.

In March the government announced a review of its decision to relax the cannabis laws. rally

TERRY HAMMOND
Hi I'm with a mental health charity - Rethink - we're just asking people whether they know anybody who's had problems with cannabis

JUSTIN ROWLATT
Terry Hammond believes cannabis can have a serious effect on the mental health of many young smokers.

UNIDENTIFIED SPEAKER
"he does get a bit angry, a bit aggressive" "give that to his mum."

JUSTIN ROWLATT
He's absolutely convinced it's what caused his son Steve to develop schizophrenia.

TERRY HAMMOND
cannabis was the trigger... he was smoking the cannabis, he was beginning to binge on it It could well be that Steve may have what they call a vulnerable personality and I think that's probably the case and that he may be somebody at risk, but I am pretty sure that had he not taken cannabis, he would not have developed schizophrenia, I have got no doubt about that.

JUSTIN ROWLATT
Some psychiatrists are also persuaded of link the between cannabis and schizophrenia - the most serious form of psychosis. Long-term psychotic illness is rare - about one in a hundred of us will experience it at some time in their lives*.

PROFESSOR ROBIN MURRAY INSTITUTE OF PSYCHIATRY
Traditionally people with schizophrenia- many of them have had problems right throughout their life, they've maybe some developmental problems, or as children they've been to see child psychiatrists and then they gradually slipped into developing schizophrenia.

JUSTIN ROWLATT
Professor Murray has been treating patients in South London for over twenty years. He says in the late eighties the sort of patients he was seeing changed.

PROFESSOR ROBIN MURRAY INSTITUTE OF PSYCHIATRY
I was seeing patients with the psychosis who had been very normal as children, children who had been great at school, who'd been academically successful, who'd been very sociable, who'd been very sporty, really kids that every parent would be proud of and wouldn't think there were any problems and then they went psychotic. Now that is much more unusual.

JUSTIN ROWLATT
The one thing these patients had in common was cannabis. Professor Murray had always believed - like most psychiatrists - that cannabis did not play a role in mental illness. Many parents questioned that.

PROFESSOR ROBIN MURRAY INSTITUTE OF PSYCHIATRY
I still see parents who say that their son, or their daughter less commonly, had been seeing a psychiatrist and they've always said to the psychiatrist, could it be the cannabis, and the psychiatrist has said no, no, I don't think it is anything to do with that.

JUSTIN ROWLATT
The view in the profession was that people who are prone to psychosis take cannabis to calm themselves down. Professor Murray and his colleagues took a closer look at some of their patients.

PROFESSOR ROBIN MURRAY INSTITUTE OF PSYCHIATRY
We found that the people who kept taking cannabis were three times more likely still to be psychotic, still to be hearing voices, still to be being deluded after 4 years. So we got to thinking, maybe if once you're psychotic it makes you worse, could it actually have contributed to developing psychosis in the first place?

JUSTIN ROWLATT
James started smoking dope at the age of 14. Pretty soon he was using it almost daily) He became anxious and paranoid. James then made a common mistake. He thought that smoking more cannabis would make him feel better.

JAMES
I thought to get to sleep I could smoke dope because dope relaxes me. I bought an eighth and I smoked the whole thing and then I was walking back to my dad's and I started hearing voices and they were like shouting at me.

JUSTIN ROWLATT
What sort of things were they saying?

JAMES
They were saying like, oi come over here, I can help you and I was really paranoid then, I started thinking that the police were after me and I thought that they were like invisible policemen with guns and they were holding them to my back.

JUSTIN ROWLATT
Did you know that they were in your head?

JAMES
I thought they were real... And I was going pretty crazy. And I kept on crossing the road and I kept on like speaking to myself, saying I'm right and then I'd cross on to the right side and I'd go no, no, no, I'm wrong.

MOTHER
As soon as I opened the door up and James came in, I could tell there was something really, really wrong because he wasn't even standing still on the spot, he was kind of like jumping up and down, he couldn't keep his feet still and it was quite a job for him just to take his shoes off when he got inside

JUSTIN ROWLATT
James was having his first psychotic episode and was referred to a psychiatrist.

DR BILL YOUNG, ADOLESCENT PSYCHIATRIST
Psychosis is really a description of a mental state of an individual who, in some way looses their connection with reality - So they may develop abnormal perceptions and experiences and on the basis of those abnormal experiences they may develop abnormal beliefs and these experiences and beliefs drive their behaviour and obviously make them behave in ways which can be unpredictable, can be bizarre to other people and can at times be violent.

JAMES
I started thinking that my TV was speaking to me - and I saw like a prince in it and I thought that was Lucifer, which was like the real name for the devil and I thought that was like my story

MOTHER
and then he started to say that he was Lucifer the devil and he um┐ his whole face changed and he put on this awful voice saying that he was Lucifer in this horrible hissing voice.

JAMES
I was starting to think that the movie was changing, the words that I was saying was making it change and I thought like people were like changing it on computers to the story of Lucifer and I thought that was like the true story of Lucifer which was my film, because I thought I was the devil.

MOTHER
I didn't know whether he might try and attack me if he thought he was the devil. But basically it went on like this sort of like all through the night with the loud telly, you know... and the voices and turning the lights up bright, you know... all through the night, and I phoned my sister about 6 o'clock in the morning and said - laugh - help

JUSTIN ROWLATT
James - who's now 16 - ended up here - in a secure adolescent mental health unit.

DR BILL YOUNG, ADOLESCENT PSYCHIATRIST
The majority of young people who are admitted have a psychotic illness of some kind and looking into the histories of those young people, cannabis use would be present and probably playing a significant role in the development of their problems in at least 75% perhaps as much as 90%

JUSTIN ROWLATT
Cannabis would play a significant role in more than three quarters of the people that you see here?

DR BILL YOUNG, ADOLESCENT PSYCHIATRIST
Yes, yes and in fact it's unusual for us to see a young person with a severe psychotic illness where there has been no exposure to cannabis at all.

JUSTIN ROWLATT
Researchers around the world have followed thousands of young people as they grow up. Their work suggests the likelihood of experiencing psychotic symptoms is increased if you are a heavy cannabis smoker during your teens. Dope smokers were found to have an almost one in ten chance of developing symptoms of psychosis by their mid-twenties. Not only that. The earlier you start and the more you smoke the higher the risk of illness.

JUSTIN ROWLATT
Some of the effects that dope smokers savour are actually very similar to the patterns of thought of patients with psychosis.

DR PHILIP ROBSON CANNABINOID RESEARCH INSTITUTE
Part of the intoxication experience may well include psychotic type symptoms, and indeed some people may really enjoy that.

CANNABIS SMOKER
It gives some something I'm quite addicted to - and that is the feeling of aha!- it allows me to step back and see the big picture - it puts you into a very analytical -not in a sort of Dr Spock kind of that is illogical captain- but examining things and analysing maybe the currents that go with it.

DR PHILIP ROBSON CANNABINOID RESEARCH INSTITUTE
I've had people describe to me -thoroughly enjoying absolutely psychotic symptoms like for example smelling colours or seeing sounds.

CANNABIS SMOKER
Cannabis seems to connect parts of your mind in ways that it doesn't normally do and then you can really make sense of things.

DR PHILIP ROBSON CANNABINOID RESEARCH INSTITUTE
A psychotic symptom is really a misinterpretation of reality. A psychosis is a bunch of those symptoms which causes me to lose touch with reality.

JUSTIN ROWLATT
For most cannabis users, the symptoms of psychosis disappear as the drug wears off... Most but not all.

JUSTIN ROWLATT
When James started smoking dope it was fun. Sometimes he'd smoke with his dad - his parents had separated. His dad would even buy cannabis for him.

JAMES
I was smoking it virtually every day and I was going to say yeah, I'll keep on smoking it 'till I die, I love it, and all this stuff. I even got a tattoo of it.

MOTHER
I found out that he started smoking and I wasn't very pleased about that. I didn't actually know much about cannabis- I wasn't sure how worried I was supposed to be.

JUSTIN ROWLATT
Looking back, his mother now thinks the first subtle signs of James' problems might have been apparent quite soon after he started smoking.

MOTHER
His personality just seemed to... change really. He was getting a lot more serious - he'd lost his enthusiasm for anything. - he'd lost his enthusiasm - so it was hard to tell whether that was just part of being a normal teenager or whether there was something more to it.

JUSTIN ROWLATT
She was worried enough to take James to the local GP.

MOTHER
The doctor said that he'd be more worried about James smoking normal cigarettes than he would about him smoking cannabis, that James was basically the normal teenager

JUSTIN ROWLATT
Scientists are beginning to understand more about what it is to be a normal teenager. It had been assumed that by the teens, brains are pretty much fully formed. We now know the adolescent brain is a work in progress - rewired right through the difficult teenage years.

DR BILL YOUNG, ADOLESCENT PSYCHIATRIST
Many cells are making new connections and branching out, other cells which don't seem to be needed are literally being pruned and these changes are occurring particularly in the parts of the brain that were responsible for reasoning and problem solving and it's those areas that seem to be particularly vulnerable in early adolescence, vulnerable to exposure to drugs, including cannabis.

JUSTIN ROWLATT
This is one of the few farms licensed to grow cannabis in Europe. These men breed cannabis for scientific research and for medicinal use.

JUSTIN ROWLATT
Cannabis contains hundreds of different chemicals - sixty of which are unique to the plant.

CANNABIS PRODUCER
Here, we are looking to breed a stable quality. We want the same amount of THC.

JUSTIN ROWLATT
THC is the chemical that gives dope smokers the high. Until recently very little was known about how THC - the active ingredient of one of the most widely used recreational drugs in the world - actually works on the teenage brain.

The big breakthrough came in the nineteen eighties. It was discovered that cannabis binds to receptors found throughout the brain and that it mimics naturally occurring brain chemicals. So this network of receptors was named the cannabinoid system after the drug. Professor Yasmin Hurd has been studying the cannabinoid system for a decade. CAPTION: PROFESSOR YASMIN HURD, KAROLINSKA INSTITUTE

JUSTIN ROWLATT
So, Yasmin, this is a human brain?

PROFESSOR YASMIN HURD, KAROLINSKA INSTITUTE
Yes it is.

JUSTIN ROWLATT
So, which parts of the brain does cannabis affect?

PROFESSOR YASMIN HURD, KAROLINSKA INSTITUTE
Cannabis affects a large part of the brain, because cannabinoid receptors, where Cannabis works, are located throughout the cortex - all of this is our cortex - this is our cerebellum important for motor coordination and cannabinoid receptors are all over the brain.

JUSTIN ROWLATT
So cannabis really has a very profound effect on the brain. Affecting almost all the brain functions?

PROFESSOR YASMIN HURD, KAROLINSKA INSTITUTE
Exactly. So, cannabis affects learning and memory, it affects pain regulations, motor function, emotional regulation, so it's because the cannabinoid receptors are one of the most abundant receptors throughout all of the brain.

JUSTIN ROWLATT
Using the latest scanning technology scientists can see how cannabis stimulates a key brain chemical, dopamine.

It's known as the "reward" chemical. When we eat, have sex or, take drugs it is dopamine that transmits feelings of pleasure in the brain. It is now recognised that dopamine has other functions too.

PROF ROBIN MURRAY INSTITUTE OF PSYCHIATRY
Dopamine is the chemical which grabs your attention, so if suddenly something exciting happens, then this is mediated by dopamine, you pay attention to something when dopamine is released. And in psychosis what people find is that everything seems to catch their attention, that a little noise or the way that somebody grimaces or somebody new comes in the door, or they hear something on the television, all of these things seem to be tremendously important and it's because the dopamine makes them important and then they think, gosh, these are really important to me and begin to connect them, they think one is connected to the other, they think that television is talking to them, they think the neighbours are conspiring against them. And it's the dopamine release that causes people to develop these delusions.

JUSTIN ROWLATT
What role does cannabis play in the dopamine system?

PROF ROBIN MURRAY INSTITUTE OF PSYCHIATRY
Well the cannabis just revs up the dopamine.

JUSTIN ROWLATT
Mark has been a heavy dope smoker since his early teens. He's developed some alarming symptoms and has been referred to a psychologist.

MARK
... when I was trying to get to sleep and I closed my eyes and put my head down and I can just.. I could hear my mates talking, it was two of my mates just talking to each other.

JUSTIN ROWLATT
What, they were having a conversation...

MARK
Yeah.

JUSTIN ROWLATT
in your head.

MARK
They were there talking to each other about the most randomest things... talking about skateboarding or something. And then it started going on to... I'd open my eyes and I could see people standing at the bottom of my bed and stuff.

JUSTIN ROWLATT
Mark and his friends are in their late teens. We've have disguised their identities - not because they are dope smokers - but because Mark has been identified as being at risk of developing psychosis. His mother and doctors want to protect him from the stigma some still attach to mental illness.

JUSTIN ROWLATT
did you ever feel paranoid?

MARK
Yeah, paranoid just pisses you off because you can't do anything, you can't relax or anything without being worried that someone is there, constantly looking over your shoulder. I couldn't walk down alleyways with my headphones in and everything, I'd always look behind me and everything.

JUSTIN ROWLATT
Because you'd feel nervous.

MARK
Yeah. At some points I just took my headphones out and ran... just like ran and...

JUSTIN ROWLATT
Although there was no one there.

MARK
No one's there but you just run anyway because it's proper freaky.

JUSTIN ROWLATT
Most people who smoke dope in their teens do not develop psychosis. So why do some? Professor Murray wondered whether those that do are in some way more sensitive to the drug. Might it have something to do with their genes?

PROF ROBIN MURRAY INSTITUTE OF PSYCHIATRY
We think that cannabis has its effect on the dopamine system therefore we wondered whether people who differed in the genes which metabolise dopamine might contribute to the susceptibility.

JUSTIN ROWLATT
The results suggest that differences in the genes that help regulate how that key brain chemical is broken down do indeed affect how likely teenage cannabis smokers are to suffer mental illness.

As someone who smoked cannabis in my youth, I wondered whether I might have been at risk. A simple blood test would help me find out. I am a recreational smoker- which Panorama didn't know when I was asked to work on this story. These days I only smoke tiny amounts. When I was fifteen it was different. I smoked a lot of dope at weekends and sometimes became very paranoid. So much so that I stopped using it for many years.

so that goes off to the laboratory now?

The results of my blood test would not be back for a couple of weeks.

JUSTIN ROWLATT
For some young smokers cannabis is woven deep into their social lives.

BOY 2
To be honest, I reckon like I'm addicted to it. Not physically addicted, like mentally addicted. Like if I don't have a spliff. I'll try and find ways of getting one.

JUSTIN ROWLATT
That sounds like an excuse to me.

BOY 2
Yeah. (laugh)

JUSTIN ROWLATT
Cannabis is not physically addictive but users can become very dependent on it. Those who work with dope smokers know how the drug can come to define an adolescent identity.

DR AMANDA SKEATE, CLINICAL PSYCHOLOGIST, EARLY INTERVENTION TEAM
It grows more into the whole social circles. So all their friends will smoke cannabis. So if they want to go out and - and - and socialise, they can only do that with somebody else who will also want to smoke cannabis.

BOY 3
Yeah. Like the majority of people I know smoke and so...

JUSTIN ROWLATT
So if you were with your mates and you said look I don't want a spliff, I mean you'd sort of stand out a bit.

BOY 2
Yeah, you'd be the only one not smoking.

DR AMANDA SKEATE, CLINICAL PSYCHOLOGIST, EARLY INTERVENTION TEAM
And the longer they've smoked, the longer they've been in that social circle, the more reduced like their - their social network will be.

BOY 3
Even if you wanted to not smoke it's quite awkward because you don't have to ask for a spliff, it just gets passed to you.

JUSTIN ROWLATT
So in fact it's an active... you have to actively opt out.

BOY 3
You have to say "No, I'm not having any".

JUSTIN ROWLATT
And then everybody

BOY 3
Everybody will be like... oh... and then they'll roll another one and pass at you like first or something, because they want to see if you'll crack.

JUSTIN ROWLATT
Dr Skeate has told Mark to stop smoking.

BOY 2
Amanda says if I keep on smoking cannabis there's a chance I can develop schizophrenia.

JUSTIN ROWLATT
Doesn't that give you a sort of pause for thought?

BOY 2
Yeah, it helped me cut down a lot more, but I mean... I don't know, cannabis is... it's like a hobby now, I've got used to smoking it that much that it's just a force of habit.

DRUG WORKER
what does it look like? This is cannabis bush...

JUSTIN ROWLATT
At this school in Kent they are trying to educate children about the dangers of all drugs - including cannabis. The more they know, the better equipped they'll be to resist peer pressure. The school is worried about the latest research. Many children know how easy cannabis is to come by.

UNIDENTIFIED SPEAKER
It's not very difficult if you know someone who takes cannabis and you could probably just ask him. If you know friends of friends then its quite easy to get hold of it.

JUSTIN ROWLATT
how many think you could get hold of it if you wanted to?

JUSTIN ROWLATT
But the children are confused about the legal status of the drug.

DRUG WORKER
can you have a small amount - enough for just one joint - is it allowed? hands up if you think it is? (waits for show of hands) very confusing isn't it. It's a class C at the moment - but if you're caught with it at your age - under 18 - it reverts back to a class B. It is illegal - that means you cannot have it.

JUSTIN ROWLATT
Many young smokers boast they always smoke skunk - this is one variety. Skunk is popular because it is strong. It has been claimed that the availability of skunk may contribute to development of psychosis in young smokers.

CANNABIS SMOKER
You used to get strong varies of weed that were pretty strong and you'd go Wooah! and you could only get them sometimes and it was great - now they've cross bred those and they've got 20% THC content in them - ridiculously strong - way too strong for me I'd have to water that down, way down, if I want to carry on being a responsible person.

CANNABIS SMOKER
That's what all these kids don't realise they get a ten pound bag of weed and they put it in one spliff and they get ooah catatonic and of course that ridiculous

JUSTIN ROWLATT
Skunk is strong because it has been cross- bred for its high THC content which is boosted by being grown under lights. How many young smokers actually use skunk is another matter. It is expensive and the best estimates suggest that skunk varieties account for just 15 per cent of the UK market.

JUSTIN ROWLATT
It is frequent heavy use that seems to be the real problem. We didn't have to spend very long with Mark and his friends before that became clear.

JUSTIN ROWLATT
so what effect do you think all this cannabis has had on you?

LADS
not being able to sleep. Yeah, it helps to sleep sometimes. I occasionally get like random voices in my head and...

JUSTIN ROWLATT
What, you actually hear voices?

LADS
Actual voices of... I've had a conversation with myself..

JUSTIN ROWLATT
Speaking out loud.

LADS
Speaking out loud.

JUSTIN ROWLATT
don't you find that worrying?

LADS
I don't really find it too worrying but I'm still 99.9% sane.

JUSTIN ROWLATT
you're on medication for your depression.

BEN
Well except for I'm on medication for depression.

JUSTIN ROWLATT
It can be hard for anyone to acknowledge that they may have symptoms of mental illness. Harder still in the difficult adolescent years. It becomes even more difficult when, like Mark and his friends, you get stoned a lot and so become accustomed to having strange thoughts and strange experiences.

DR AMANDA SKEATE, CLINICAL PSYCHOLOGIST, EARLY INTERVENTION TEAM
A certain amount of, if you like, unusual experiences are accepted within the group. What our clients find it hard then is that they've got much more severe problems or they're also getting paranoid two days later when they haven't had any cannabis or when they're completely on their own or in the middle of the night. And it's really difficult to say to their group, you know, I - I don't want to smoke it and, you know, I'm - I'm beg - beginning to get mentally unwell.

JUSTIN ROWLATT
This confusion and denial may help explain why many young people, their parents and their doctors do not spot the warning signs until it is too late.

JUSTIN ROWLATT
It is time to find out the results of my gene test. It would show if the gene that helps control the breakdown of the reward chemical dopamine makes me particularly sensitive to cannabis. What has happened to someone in my family has made the results even more resonant.

During the course of making this programme a relative of mine was hospitalised with a psychosis associated, it appears, with his teenage cannabis smoking, and a lot of people we've spoken to have said they know people who this has happened to too. How widespread do you think this is?

PROF ROBIN MURRAY INSTITUTE OF PSYCHIATRY
I don't think we know how widespread, but like you, I've been surprised that it's not just people who develop a full-blown psychosis but out in the population at large there are lots of people who have a brief paranoid episode. But I think one of the useful things about this programme might be that you would get better information or that it would become more widely known that this is the case.

JUSTIN ROWLATT
His studies show that genes do seem to play a role in how sensitive people are to the drug. In fact, for one in four people the chance of developing psychosis is increased tenfold if they are frequent cannabis users during their teens.

Well Robin I've got the test result here. I don't know the result so let me just have a look. Well that is unintelligible to me.

PROF ROBIN MURRAY INSTITUTE OF PSYCHIATRY
Well this actually shows you that you would be one of those individuals who are relatively unlikely to go on to develop psychosis.

JUSTIN ROWLATT
But genetic makeup doesn't offer any guarantees. The genes that help regulate the reward chemical dopamine in the brain are just one factor in the development of psychotic illness. Age is another.

JUSTIN ROWLATT
What about adult cannabis smokers?

PROF ROBIN MURRAY INSTITUTE OF PSYCHIATRY
I think the evidence is that the risk for adults of developing psychosis is not so great. In fact, you're actually getting through the major risk ages for developing schizophrenia because it's a disorder of those between adolescence and about 30.

JUSTIN ROWLATT
What Professor Murray and the other scientists have done is use population studies to show that heavy cannabis use in your teens can increase the probability of developing psychosis. What they have not proved is that cannabis actually causes psychosis. To do that would require evidence that using cannabis physically alters the brain...

JUSTIN ROWLATT
... which is precisely what scientists in Sweden believe they have done. They exposed adolescent rats to THC - the chemical gives dope smokers the high - and found long-term changes in their brains.

JUSTIN ROWLATT
They also gave rats THC in adolescence or while their mothers were pregnant. They wanted to see if it would affect their behaviour. The researchers tested how likely the rat they were to use a switch to give themselves small doses of heroin.

PROFESSOR YASMIN HURD, KAROLINSKA INSTITUTE
We saw that animals that were exposed during development took more heroin as adults. They were also more sensitive to stress - in that they took more heroin when they got just even very mild stress, and they were sensitive to lower - even lower - concentrations of heroin, than animals... than normal animals that never got THC during development.

JUSTIN ROWLATT
As if there was some sort of predisposition to take heroin, having been exposed to cannabis, to THC, in their early lives.

PROFESSOR YASMIN HURD, KAROLINSKA INSTITUTE
Exactly. I would call it more of a risk. So they increased their risk factor, the vulnerability for taking more heroin, due to the early exposure to THC.

JUSTIN ROWLATT
Professor Hurd is a leading brain scientist with an international reputation. Her work may reignite a long-standing debate about cannabis - that it can make users more likely to use other drugs. But the implications of her THC work are much wider than that. She found physical changes in the rats' brains - changes that she has also found in pre-natal human brains exposed to cannabis.

PROFESSOR YASMIN HURD, KAROLINSKA INSTITUTE
So if we cut the brain in half and we will look at just one hemisphere

JUSTIN ROWLATT
Using an adult brain, she showed me the area that had changed.

PROFESSOR YASMIN HURD, KAROLINSKA INSTITUTE
Was we saw that - in the rat models - we saw an area that's called the nucleus accumbens which is very, very highly associated with reward - it is considered part of the reward circuit, the central reward system in the brain - that that is highly changed

JUSTIN ROWLATT
The brain has changed?

PROFESSOR YASMIN HURD, KAROLINSKA INSTITUTE
The brain has changed.

JUSTIN ROWLATT
What has changed is the central reward circuit - the dopamine system. That's the system that may provide the link between cannabis and pyschosis.

PROF ROBIN MURRAY INSTITUTE OF PSYCHIATRY
If she finds long-lasting changes in these areas, that suggests that the cannabis has altered the dopamine system and so that the individuals will be more sensitised than next time they encounter a drug like cannabis.

JUSTIN ROWLATT
So this takes us towards a proof of what has been a hypothesis underlying your work - that cannabis does have long-term changes on the dopamine system.

PROF ROBIN MURRAY INSTITUTE OF PSYCHIATRY
Yes, and it's not so unusual for a drug to have long lasting changes. We know that amphetamine or heroin or even alcohol can produce long-lasting changes in the neurochemistry of the brain. So this is really rather as one would have expected, but thank goodness somebody has now done it.

JUSTIN ROWLATT
Professor Hurd's work has not yet been published or replicated but she says it confirms the unique vulnerability of the teenage brain.

PROFESSOR YASMIN HURD, KAROLINSKA INSTITUTE
Absolutely, because the circuits, the synapses, are not all formed yet so a drug has more ability to impact the final formation of the brain┐

JUSTIN ROWLATT
So, if you start smoking at twelve, the chances are, you will not be the person you would have been, had you not smoked cannabis.

PROFESSOR YASMIN HURD, KAROLINSKA INSTITUTE
Correct. Does it mean that you are a better or a worse person? No. But you are definitely a different person.

JUSTIN ROWLATT
It is a controversial conclusion. Some will argue about the full implications of the changes she has identified.

JUSTIN ROWLATT
Sixteen--year-old James had recovered well in the secure adolescent unit. Medication had helped control his psychosis.

JAMES
I got better after a month and I went home for Christmas.

JUSTIN ROWLATT
What did the doctors say to you?

JAMES
Don't smoke dope ever again.

JUSTIN ROWLATT
What did you do?

JAMES
Went out and smoked dope.

MOTHER
As I started to go upstairs the music seemed to be getting louder and louder, and it was really very loud in here and it was very haunting walking in here. James was talking to himself quite clearly very confused again.

JUSTIN ROWLATT
Psychotic.

MOTHER
Yeah, as I entered the room I really couldn't believe that everything was going wrong again so soon

JAMES
This wired feeling swept over me and I thought I was the lead singer of a band called Danny filth and I stated to like singing to it... screaming and roarin'.

JUSTIN ROWLATT
There may still be doubt about whether the illness James suffered was caused by cannabis. But there is no doubt about cannabis causing relapses in psychotic patients. James was taken back to hospital.

MOTHER
I did feel like I'd lost him because this... psychosis had taken over, part of his personality and um... part of him really it just wasn't James anymore, and that seemed to be more powerful than anything else.

DR BILL YOUNG ADOLESCENT PSYCHIATRIST
He really seemed to be inaccessible to us for quite some period of time - he would disappear into a place where he felt terrified by everything that was going on around him and really in immediate fear of his life.

JUSTIN ROWLATT
This psychotic episode was much worse - James was profoundly ill for over two months. At the unit it is a familiar pattern.

DR BILL YOUNG ADOLESCENT PSYCHIATRIST
Young people who may have used cannabis at very high levels in the past only need to be exposed to relatively small amounts to develop symptoms again. So in some ways they are exquisitely sensitive to the effects of cannabis.

JUSTIN ROWLATT
So you have to issue a very strong message, once you have had a psychotic episode, don't smoke. For the young people here there is really no question about that. It is a complete no no.

JUSTIN ROWLATT
Professor Murray believes that each relapse makes another more likely.

PROF ROBIN MURRAY INSTITUTE OF PSYCHIATRY
Many people who develop psychosis would normally recover, but the people who develop psychosis, recover and then go down the road and meet their dealer and start smoking again, they come back into the unit, we treat them, they get better, they go out, they come back in. So our wards are now full of people who are re-cycling through them because they are dependent on cannabis and they don't give up.

In south London where I work, the frequency of schizophrenia has doubled since the 1960s. And I think drug consumption is one factor.

JUSTIN ROWLATT
Steve Hammond suffers from schizophrenia. Schizophrenia is the most serious, long-term form of psychosis. Steve started smoking dope when he was about 16. He's now 27. His father says Steve knew that cannabis and his illness were intimately linked.

TERRY HAMMOND
He was heavily smoking it, he was smoking 5 or 6 a night. What he tells me now is that when he took it, he said it was actually like switching a light on, he said he instantly felt psychotic, he said I only needed to take a couple of puffs he said and it was like putting a light on.

JUSTIN ROWLATT
There is growing recognition of the role cannabis can play in mental illness but not all psychiatrists are persuaded that cannabis causes schizophrenia.

DR PHILIP ROBSON CANNABINOID RESEARCH INSTITUTE
We really do have to be careful with our terminology here. Psychotic episodes as an acute reaction to cannabis - definitely. Schizophrenia as a long-term result of smoking cannabis in your youth - not proven as yet.

Schizophrenia is a very serious diagnosis to give someone. As the most devastating psychosis, one which is involved in┐ with abnormalities of perception, emotion, thought, motivation, motor function, which devastates people's lives and has a terrible prognosis.

TERRY HAMMOND
I am totally convinced he would not have developed schizophrenia, which fundamentally has ruined his life, that's the bottom line. It has now caused brain damage, that's the reality of it and I think there are 10's of 1000's of kids out there who are damaging their brains, and you know, I think it's a public health time bomb.

JUSTIN ROWLATT
Mark knows he is taking a risk but he still finds it very difficult to stop smoking dope.

LADS
Your family is worried about you, your doctors are worried about you. Are you worried about you? You're not worried. Why not?

BOY 1
I think I can handle myself. I don't really.. I don't care about myself that much. It's more my family, hurting them and everything.

JUSTIN ROWLATT
Isn't that what all kind of adolescent boys say, they don't care about the future, they don't care about themselves, but when the future comes, you probably will care, you know what I mean.

BOY 1
Yeah, I know that like if it happens in a few years I'm going to be looking back at myself and thinking you're a dickhead for doing it. But at the moment I'm just.. I'm living in the moment.

JUSTIN ROWLATT
And is that alright? Is that a good enough explanation?

BOY 1
[nodding affirmatively]

JUSTIN ROWLATT
James has spent over four months at the secure unit since his relapse. Today he is going home.

DR BILL YOUNG ADOLESCENT PSYCHIATRIST
I would say that James is probably a different person now and probably a stronger and wiser person than he was before.

MOTHER
Its such an enormous relief to think that he was ready to come home

JUSTIN ROWLATT
I'm hoping that he can put all this horrible experience behind him cos he was so young to go through something like that

JUSTIN ROWLATT
If James can avoid cannabis his chances of making a full recovery are good.

JUSTIN ROWLATT
So how does it feel just to be able to open your front door and walk out into the world?

JAMES
It feels really good. I'm back in the real world and I'm going to stay there. I'm going down the park with all my mates maybe just have a couple of drinks just to celebrate that I'm back

JUSTIN ROWLATT
And do you reckon anyone will be smoking dope?

JAMES
May be a couple of people doing that

JUSTIN ROWLATT
Is there any chance a spliff will come your way?

JAMES
No

JUSTIN ROWLATT
So you're determined to steer clear now?

JAMES
Definitely not going to touch it again J: never smoke again?

JAMES
No.

JUSTIN ROWLATT
Young people cannot be certain what risks they take when they smoke cannabis. What we are all learning - and what teenagers need to know - is that if you start smoking the drug early and smoke it a lot - then for some, the consequences can be very serious.

You can find full details of the science and research discussed in tonight's programme by visiting our website at BBC.co.uk forward slash Panorama. You can also sign up to find out about future programmes.

(And ) If you have been affected by tonight's programme and would like details of organisations that can provide support then call the BBC Action Line on 0800 888 809 that's 08 Thousand 888 809 .

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