An expert on the health effects of cannabis says that there is growing evidence that the drug is responsible for mental health problems.
The health effects of cannabis are controversial
Professor John Henry, a consultant in toxicology from St Mary's Hospital in London, told the BBC that studies from Sweden and elsewhere pointed to an increase in schizophrenia among regular cannabis smokers.
The mental health effects of smoking cannabis are a controversial area, with any evidence of harm strongly disputed by some.
However, Dr Henry is planning to tell a conference at the Royal Society of Medicine on Monday that it appears likely that some cases of schizophrenia are attributable to the consumption of cannabis, rather than the alternative explanation that patients prone to mental illness are more likely to be drawn to use the drug.
People who want to smoke cannabis ought to be aware that it has equal effects to cigarettes on the body and worse effects on the mind
Dr John Henry, St Mary's Hospital
Dr Henry says that the strength of cannabis on sale now far outstrips the strength of the drug sold during the "flower power" era of the 1960s and 1970s.
He told the BBC: "There's no government health warning against cannabis but there are all kinds of warnings about tobacco.
"People who want to smoke cannabis ought to be aware that it has equal effects to cigarettes on the body and worse effects on the mind.
"You've got the fact that regular cannabis smokers develop mental illness.
"There's a fourfold increase in schizophrenia and a fourfold increase in major depression.
"That is something very very different from what smoking does to you.
"There's a lot of epidemiological evidence from as far apart as Sweden and New Zealand that cannabis actually causes these problems."
According to a review carried out by UK drugs information service Drugscope, evidence of long-term mental health effects of cannabis is far from clear-cut.
It points to criticism of the Swedish study mentioned by Dr Henry - and says that while cannabis consumption is increasing, the incidence of schizophrenia is not, which would suggest that cannabis may not be to blame.
It is possible, says Drugscope, that cannabis precipitates schizophrenia in people who would have developed it anyway.
Currently, the number of cannabis users in the UK is estimated at more than three million.
The drug is due to be "downgraded" this summer from a Class "B" to a Class "C" drug by the government.
This means that while possession of small quantities of cannabis remains illegal, it is not an "arrestable" offence unless there are aggravating factors, such as use of cannabis near children.
Lesley King-Lewis, chief executive of charity Action on Addiction, said: "This evidence further demonstrates that cannabis use can be dangerous.
"The public should be made more aware of the risks involved with using this illicit drug.
"As many as one in 10 cannabis users become addicted. Cannabis use is associated with cancers of the mouth, tongue, throat, oesophagus and lung and reductions in fertility, as well as with mental illnesses such as schizophrenia and depression.
"Chronic cannabis use clearly involves significant costs to society as well as to the individual user. Many of these problems are shared with alcohol and tobacco.
"Therefore more research is needed to provide effective education highlighting the relative dangers of different substances and targeting those most at risk."
Chief Executive of the British Lung Foundation (BLF), Dame Helena Shovelton, chief executive of the British Lung Foundation, described smoking cannabis as a "health time bomb".
She said: "Over 3 million regular users in the UK could end up with chronic lung failure.
"It is vital that the public know the damage smoking cannabis can cause."
Dame Helena said a BLF lung consultant recently gave a lung transplant to a young patient who had only ever smoked cannabis.
"Unfortunately, cases like this will become more and more common if public awareness of the dangers is not raised."