The UK Government will downgrade cannabis from a Class B to a Class C drug next week.
Cannabis will still be illegal under the changes
The move has sparked controversy not least among medical experts, many of whom believe it can trigger mental illnesses.
BBC News Online examines the evidence.
The decision to downgrade cannabis was always going to be controversial.
After all, the jury is still out on the long-term effects of smoking a joint.
There is strong evidence to suggest that smoking cannabis can harm physical health.
Smoking regularly - regardless of whether it is tobacco or cannabis - can increase the chances of developing lung cancer and other diseases.
CAN IT TRIGGER DEPRESSION?
Scientists in Australia reported that girls who smoked cannabis were five times more likely to suffer from depression. Their findings were based on a study of 1,600 teenagers over seven years.
Source: British Medical Journal
However, research published two years ago suggested smoking cannabis may be more harmful than cigarettes.
Tar in cannabis cigarettes can contain as much as 50% more cancer-causing carcinogens than tobacco.
What's more, cannabis smokers tend to draw more heavily on joints than cigarette smokers - increasing their risk of cancer further.
The government scientists who recommended the reclassification of cannabis described the risk as "real".
In their report, the Advisory Council on the Misuse of Drugs said smoking cannabis can increase the risk of "bronchitis, asthma and lung cancer as well as disorders of the heart and circulation".
This is one of the reasons why if the UK government allows the use of cannabis for medicinal purposes, it will come in pill form and will only contain the active ingredients of the drug.
While the risks to physical health are relatively clear cut, there is a debate about the impact of cannabis on mental health.
CAN IT CAUSE SCHIZOPHRENIA?
Swedish scientists said men who smoked cannabis were 30% more likely to develop schizophrenia. Their findings were based on a study of more than 50,000 men who smoked the drug in the 1960s.
Source: British Medical Journal
Some experts believe that smoking cannabis can trigger a range of mental health problems, from depression to psychoses.
However, others disagree saying there is only a risk for those people who are already ill.
In its report, the advisory council, which includes some of Britain's top medical and scientific minds, said "no clear causal link has been demonstrated" to show cannabis can lead to mental health problems.
However, it acknowledged that "acute cannabis intoxication" or being stoned can lead to "panic attacks, paranoia and confused feelings".
The experts also warned that cannabis can "produce a psychotic state that may continue for some time and may require treatment with antipsychotic drugs".
When it came to people with mental illness, they were even more robust.
"Cannabis use can unquestionably worsen schizophrenia and other mental illnesses and lead to relapse in some patients," they said.
ARE TEENAGERS MOST AT RISK?
A study carried out in New Zealand found evidence to suggest that people who smoke cannabis as a teenager may increase their risk of developing schizophrenia.
Researchers based their findings on a study of 1,000 people in their early 20s. Source: British Medical Journal
Nevertheless, the council insisted that cannabis should be reclassified.
Until now, cannabis has been classed alongside drugs like amphetamines. It will now be grouped alongside steroids and tranquilisers.
The advisors were worried that keeping cannabis in Class B ran the risk of cannabis users thinking "that if they have no harmful effects from cannabis then other Class B substances will be equally safe".
However, other experts are concerned the move sends out the wrong message to the public.
Cliff Prior, chief executive of the mental health charity Rethink, is against it.
"People hear that cannabis is no longer a serious drug, that the police won't make arrests except in the most serious circumstances and that government is willing to downgrade it. People hear the message that cannabis is risk-free."
WILL USERS GO ON TO HARDER DRUGS?
Researchers in the United States say there is no evidence that people who take cannabis go on to take harder drugs. Their claims were based on a study of data from the National Household Survey on Drug Abuse between 1982 and 1994.
Dr Peter Maguire of the British Medical Association shares that view.
"The BMA is extremely concerned that the public might think that reclassification equals 'safe'. It does not."
Professor Robin Murray, one of the country's top psychiatrists, says there is growing evidence that people who regularly smoke cannabis are putting themselves at risk.
"We and about five other studies in other countries have shown that if you start taking cannabis early and heavily - say you're taking cannabis daily by age 18 - then you are about seven times more likely to develop schizophrenia than the rest of the population.
"You have to keep taking a lot every day to develop that sort of risk but psychiatric units like mine are full of people who would not have gone psychotic, who would not have developed schizophrenia, if they did not smoke cannabis."
Professor John Henry, a toxicologist at St Mary's Hospital in Paddington, grabbed headlines last year when he warned about the risks of smoking cannabis.
WHO SMOKES CANNABIS?
27% of 16 to 24 year olds
14% of 25 to 34 year olds
4% of 35 to 59 year olds
Source: British Crime Survey 2002 (Percentage of people who smoked cannabis during the previous year)
Addressing a conference in London, he said he was convinced the drug can cause mental illness.
"Regular cannabis smokers develop mental illness. There's a four-fold increase in schizophrenia and a four-fold increase in major depression," he said.
'Not as harmful'
However, others have yet to be convinced. Frank Warburton, acting chief executive of DrugScope, supports the decision to downgrade cannabis.
"Cannabis is not as harmful as other Class B drugs," he says.
"While we agree that there may be link between cannabis and mental illness, we would argue against the simple assumption that cannabis causes mental illness.
"If someone had a pre-existing condition, then cannabis may exacerbate it. That is not the same as saying cannabis causes mental illness."
Mark, who first used cannabis when he was 12, said he backed the decision to downgrade cannabis.
"Cannabis is nothing. It doesn't cause any problems. It doesn't cause any violence.
"People don't have to go out and break the law to fund it. It is cheap to buy. I don't think this will cause any problems."