Mental health hospital admissions in England due to cannabis have risen by 85% under Labour, figures show.
There are over 2m regular users of cannabis
In 1996-7, there were 510 admissions, rising to 946 in 2005-6, data obtained by shadow health secretary Andrew Lansley revealed.
Over the last five years alone there was a 65% rise, with experts saying the figures were "the tip of the iceberg".
The government said it had been clear on cannabis - it was illegal and should not be used.
Cannabis is the most widely used illegal drug in the country with over 2m regular users.
The figures obtained from Health Minister Rosie Winterton in a written House of Commons answer are for patients admitted to hospital in England because of a mental or behavioural disorder due to the use of cannabis.
Admissions are not the same as patients, so one patient may have been admitted more than once.
The figures include people with a chronic addiction to cannabis, people with an acute cannabis psychosis as well as those with cannabis-related schizophrenia.
But experts say many more cases may be missed or diagnosed simply as a mental health disorder instead.
Professor Robin Murray, professor of psychiatry at London's Institute of Psychiatry and member of the Royal College of Psychiatrists, said: "There is no doubt that cannabis-related psychiatric problems have increased substantially.
"This might be down to better recognition, but I would say these figures are just the tip of the iceberg. Its only more recently that psychiatrists have understood the importance of cannabis use."
He said cannabis use was a contributing factor in up to 10% of schizophrenia cases, yet this was under-recognised.
"There are probably 1,500 new cases of cannabis-related schizophrenia a year," he said.
Paul Jenkins, chief executive of the Rethink mental health charity, said: "These figures show that there is an urgent need for a properly funded campaign to help young people realise cannabis use is risky."
Marjorie Wallace, chief executive of the mental health charity SANE, said: "Psychiatrists have been telling us that 80% of patients admitted with their first episode of psychosis have been taking the drug.
"We strongly urge the government to heed the growing evidence and take urgent action to warn young people that some of them are risking lifelong mental illness - that they are playing Russian roulette with their minds."
Scientists have found that some of the active ingredients in cannabis can have an impact on the symptoms of many diseases, including asthma, glaucoma and muscle spasms, as well as loss of appetite and nausea due to AIDS and chemotherapy treatment.
Indeed, cannabis-based pain-relief drugs have been licensed for MS patients.
Mr Lansley added: "Awareness of the link between mental illness and cannabis has increased over recent years, as has the strength of the drug.
"Both these factors have contributed to the sharp increase of hospital admissions on mental health grounds.
"That's why the Conservatives have opposed the downgrading of cannabis and pledged to have it reclassified."
At present, cannabis is a class C drug but there have been calls to move it back to class B.
A Department of Health spokeswoman said: "Our message is clear - cannabis is harmful, is an illegal drug and should not be taken.
"Our policy on cannabis is in line with the drugs strategy, with emphasis on enforcement, prevention, education, and treatment; evidence shows that cannabis use is falling across all age ranges."