By Michelle Roberts
Health reporter, BBC News
Cannabis is an illegal drug in most countries
People who smoke potent skunk are more at risk of psychotic illnesses such as schizophrenia than those who use other types of cannabis, scientists suspect.
According to new research, regular users double their risk of psychosis but heavy skunk users increase theirs seven-fold.
UK experts have a theory it is down to skunk's composition - it contains more of the chemical that gets users stoned.
The work is published in British Journal of Psychiatry.
The findings come only weeks after the UK's chief drugs adviser was sacked after he criticised the government's decision to reclassify cannabis up to Class B from C.
The authors of the latest research from the Institute of Psychiatry were quick to stress that their work is merely to inform.
And they point out that drug use only accounts for the minority of cases of psychotic illnesses - somewhere between 10% and 15%. Other risk factors, such as family history of mental health problems, play a far bigger part.
But they say cannabis, and particularly stronger skunk, should be considered a potential health hazard in a similar way to alcohol.
Just as downing a bottle of whisky a day is riskier than drinking half a glass of wine each evening with your dinner, smoking strong skunk every day poses a greater threat than smoking less potent types of cannabis every now and then, they say.
When Dr Marta Di Forti and colleagues at the Institute of Psychiatry screened 280 patients admitted to their hospital with psychotic symptoms for the first time, they found most - nearly 80% - were heavy skunk users.
They also questioned healthy controls of a similar age and social background, who they recruited through newspaper ads and the internet, about their personal drug use.
They found no real difference between the two groups in whether they had ever used cannabis or their age at first use.
But the patients with psychosis were twice as likely to have used cannabis for longer than five years, and over six times more likely to use it every day.
Moreover, among those who had used cannabis, patients with psychosis were seven times more likely to use skunk than controls.
The experts believe skunk is particularly damaging because it contains more THC.
This is the main psychoactive ingredient and has been shown to produce psychotic symptoms like hallucinations and delusions in experiments.
Unlike skunk, hashish - cannabis resin - contains substantial quantities of another chemical called cannabidiol or CBD and research suggests this can act as an antidote to the THC, counteracting its psychotic side effects.
Dr Di Forte said their findings were concerning, particularly as skunk has come to dominate the UK cannabis market in recent years.
"Public education about the risks of heavy use of high-potency cannabis is vital."
She said more far work was needed to assess the exact risks of smoking different types and quantities of cannabis.
Experts do know that the risks go up with both duration and amount of use.
A spokesman from The Legalise Cannabis Alliance UK said: "We don't need to worry about the health harms of people smoking cannabis per se, whether it is skunk or not.
"What is a concern is that as a result of prohibition some dealers put other stuff into the cannabis they sell that may be damaging. I've heard of lead and glass being put in it."
Chris Hudson, addictions expert at the charity Frank, said: "You never truly know what you're getting and stronger cannabis, such as skunk, can increase the chance of suffering a nasty reaction."
A Home Office spokesman said: "The reclassification of cannabis as a Class B drug was partly in response to emerging concerns about the growing use of stronger strains of cannabis, such as skunk, and the harm they may cause to users' mental health.
"We remain determined to crack down on all illegal substances and minimise their harm to health and society as a whole."
Marjorie Wallace of the mental health charity SANE said: "We receives daily evidence that the long-term use of skunk, with its specific chemical make-up, can trigger frightening psychotic episodes, cause relapse and may bring about mental conditions such as schizophrenia.
"It can also rob developing young minds of their potential and wreck their futures and those of their families."