Far from being a relatively harmless "soft" drug, cannabis can drive people temporarily insane as well as harming the heart, lungs and immune system, British scientists said today.
Studies showed that the drug can have serious effects on both mental and physical health.
Dr Andrew Johns, from the Institute of Psychiatry in London, said regular cannabis use led to acute psychological problems in a high proportion of people.
One study found 15 per cent of cannabis users identified psychotic symptoms such as hearing voices or irrational feelings of persecution.
A number of reports, reviewed by Dr Johns in the British Journal of Psychiatry, suggested the drug could induce psychosis in people with no history of severe mental illness.
Heavy cannabis use could lead to a state resembling acute schizophrenia. In this case the drug was thought to trigger off an underlying illness.
Cannabis was also associated with high rates of other forms of mental illness, such as adjustment disorder and major depression.
It appeared to worsen the condition of people already suffering from schizophrenia, with users experiencing more and earlier psychotic relapses.
Dr Johns wrote: "An appreciable proportion of cannabis users report short-lived adverse effects, including psychotic states following heavy consumption, and regular users are at risk of dependence.
"People with major mental illnesses such as schizophrenia are especially vulnerable in that cannabis generally provokes relapses and aggravates existing symptoms.
"Health workers need to recognise, and respond to, the adverse effects of cannabis on mental health."
In another review published by the journal, Prof Heather Ashton of the University of Newcastle said cannabis affected almost every bodily system.
As well as producing severe anxiety, panic, paranoia and psychosis in high doses, it also impaired memory and concentration and had a number of physical effects.
These included heart problems that might be serious for people with pre-existing cardiac disease and suppression of the immune system.
In addition, smoking between three and four cannabis reefers a day produced the same risk of bronchitis and emphysema as 20 or more tobacco cigarettes.
Tar from a cannabis cigarette contained higher levels of some cancer-triggering chemicals than tobacco smoke.
Chronic cannabis use also appeared to cause a risk of complications in pregnancy and childbirth, said Prof Ashton.
But cannabis and its derivatives had promising potential as a treatment for a large number of medical conditions, said Dr Philip Robson, also writing in the journal.
Chemicals in the drug had been shown to have therapeutic effects in nausea caused by anti-cancer drugs, multiple sclerosis, loss of appetite, pain, glaucoma, insomnia, anxiety and depression, epilepsy, and asthma.
There was also evidence from animal studies that cannabis derivatives can protect against cancer.
PA