Cannabis smoking - besides causing harm to heart, lungs and the immune system - can lead to temporary bouts of mental illness.
Scientists report today in the British Journal of Psychiatry that regular use may make things worse for people who have mental health problems, and lead to panic attacks and anxiety in those who do not.
Andrew Johns of the Institute of Psychiatry at the Maudsley hospital in south London surveyed a number of recent studies. One found that 15% of users identified psychotic symptoms or irrational feelings of persecution. Other reports suggested the drug could induce psychosis in people with no history of severe mental illness.
Those with mental illness - living in the community, and as likely as anybody else to get hold of the drug - were even more at risk. "People with major mental illnesses such as schizophrenia are especially vulnerable, in that cannabis generally provokes relapses and aggravates existing symptoms," Dr Johns said.
"Health workers need to recognise, and respond to, the adverse effects of cannabis on mental health."
Researchers are testing cannabis as a potential medicinal drug - there are claims that it can quell nausea during chemotherapy, relieve glaucoma and stifle the pain of multiple sclerosis - but smoking marijuana also imposes a price.
Last year US researchers showed that squirrel monkeys found the drug addictive, and a Boston team reported that, an hour after inhaling, the risk of heart attack increased fivefold.
Heather Ashton, of the University of Newcastle, reports in the same journal that besides producing severe anxiety, panic, paranoia and psychosis in high doses, cannabis impaired memory and concentration.
There could be heart problems for people with pre-existing cardiac disease, and the drug also suppressed the immune system. Cannabis cigarettes could be as addictive as nicotine, and the tars from cannabis cigarettes contained higher levels of some cancer-causing chemicals than tobacco.
Smoking three or four reefers a day produced the same risk of bronchitis or emphysema as 20 or more cigarettes.
Chronic use might also cause complications in pregnancy and childbirth.
