Men who take cannabis regularly have a one in 5,000 chance of developing a disease like schizophrenia, according to one of the top drug specialists in the UK.
But Professor David Nutt, who chairs the government's Advisory Council on the Misuse of Drugs (ACMD), told Newsbeat the risk to mental health from smoking cannabis is no greater than getting drunk.
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"We've written three reports and read every paper published for the last 30 years. We do not believe the risks are going up," he said.
"When we look at the evidence, we have seen a huge increase in the use of cannabis but a fall in schizophrenia."
"Alcohol is probably more likely to cause dependence than cannabis.
"It causes brain damage through vitamin deficiency and withdrawal can lead to psychosis. Overall the mental health risks of alcohol and cannabis are not dissimilar."
Mental health impact
The government has upgraded cannabis to a Class B substance this week against the advice of the ACMD, partly because it says there is growing evidence it does damage mental health.
The drug has been closely linked to schizophrenia, an illness which can cause hallucinations, paranoid or bizarre delusions and disorganised speech and thinking.
Twenty-nine-year-old Alec Jacobs from Kent was a heavy cannabis user before developing the disease in his early 20s.
"I was hearing voices and getting anxious and depressed. I think cannabis definitely played a part in doing something to me," he told Newsbeat.
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"I was smoking a lot of skunk which I think sent me into a psychosis.
"By the age of 21, I found it hard being around people and I couldn't smoke it anymore because it wasn't doing me any good whatsoever.
"I think there is definitely a lasting effect. I still get depressed and anxious and feel sort of trapped in a way."
But Professor Nutt from the ACMD reckons the link between cannabis use and that kind of severe mental health problem is "probable but weak".
The latest research suggests the government would have to stop 5,000 men and 12,000 women from smoking cannabis to prevent a single case of schizophrenia in both groups.
"Using cannabis will tip a few people over the edge but in terms of most of the population, there isn't really a risk there," he said.
Serious crimes link
The vast majority of people with the schizophrenia tend to suffer personal problems like depression but the most extreme cases can lead to dangerous behaviour.
Marc Middlebrook, 27, was sentenced to life imprisonment last year for stabbing his girlfriend Stevie Barton to death because he believed she was part of a plot to kill him.
The court heard that he had made his mental problems worse by "stubbornly" continuing to smoke cannabis after doctors told him to stop.
Newsbeat spoke to Stevie's mother Jackie, a former psychiatric nurse.
She said she didn't blame the drug for her daughter's death.
"I always say cannabis didn't kill my daughter, Marc did," she said. "I know lots of people - doctors, professionals, nurses - who have smoked cannabis for years and do not commit crimes."
"It's no good standing there wagging your finger and saying this is wrong. People need to be able to know the facts and there is a lot of information and counter information around cannabis use at this time."
"What message are you giving to people when you downgrade cannabis to a Class C and then take it back up to a Class B? It's that you don't know what you are doing."
"Let's not turn this into a witch hunt. Let's not damn all cannabis users, that would be wrong.
"We need an adult approach. Let's get some education."
By Jim Reed
January 27, 2009