LONDON (Reuters) - Cannabis is not the harmless recreational drug many users
think it is but a dangerous substance that can cause paranoia, psychosis and
severe anxiety and panic, psychologists warned on Thursday.

A review of research into the drug that first appeared in China nearly 5,000
years ago shows it can impair perception, motor skills and reaction times
but it also has medicinal qualities for a range of illnesses.

"Cannabis affects almost every body system. It combines many of the
properties of alcohol, tranquilizers, opiates and hallucinogens," said
Professor Heather Ashton, of the University of Newcastle upon Tyne.

Ashton reviewed studies on the recreational use of cannabis, its potency and
impact on the body and brain. Her research, published in the British Journal
of Psychiatry, shows it is still popular among young people.

Sixty percent of university students in Britain have tried the drug and 20
percent use it regularly. In a recent survey some users admitted to smoking
up to 15 joints or more a day.

Although most people smoke cannabis occasionally for its euphoric feeling,
regular use has more serious side effects including impairment in memory,
attention and the ability to process complex information.

"Whether there is permanent cognitive impairment in heavy long-term users is
not clear," Ashton added.

And contrary to common belief, chronic users can develop a tolerance and
dependence on cannabis which can lead to withdrawal effects similar to those
of alcohol and opiates, she added.

Using the drug also increases their risk of developing bronchitis and
emphysema. Smoking three or four joints a day is the equivalent of about 20
cigarettes.

But in a separate report in the journal, Dr. Philip Robson, a psychiatrist
at the University of Oxford, focused on the medical benefits of the drug,
which was traditionally used to treat malaria, rheumatic pains and during
childbirth and later used for insomnia, asthma and gout.

THC, the compound in cannabis which causes the high, can relieve vomiting
and nausea in cancer patients. The drug also reduces muscle pain in multiple
sclerosis patients and stimulates appetite and cuts weight loss in cancer
sufferers.

"Cannabis itself, clearly from anecdotal reports, is useful across many
conditions. There is a certain amount of scientific evidence to back that up
but it is limited in its quality and scope because it is difficult to do
research on illegal drugs," Robson said in a telephone interview.

"There is a consensus... that these are a group of drugs that are well worth
studying in a range of medical conditions for which standard treatments are
really unsatisfactory." he added.

Robson acknowledged there were side effects but he said they must be weighed
against the unpleasant effects of toxic drugs prescribed for many medical
conditions.