LONDON, England (AP) -- Using marijuana seems to increase the chance of
becoming psychotic, researchers report in an analysis of past research that
reignites the issue of whether pot is dangerous.

Some experts say governments should now work to dispel a misconception that
marijuana is a benign drug.

The new review suggests that even infrequent use could raise the small but
real risk of this serious mental illness by 40 percent.

Doctors have long suspected a connection and say the latest findings
underline the need to highlight marijuana's long-term risks. The research,
paid for by the British Health Department, is being published Friday in
medical journal The Lancet.

"The available evidence now suggests that cannabis is not as harmless as
many people think," said Dr. Stanley Zammit, one of the study's authors and
a lecturer in the department of psychological medicine at Cardiff
University.

The researchers said they couldn't prove that marijuana use itself increases
the risk of psychosis, a category of several disorders with schizophrenia
being the most commonly known.

There could be something else about marijuana users, "like their tendency to
use other drugs or certain personality traits, that could be causing the
psychoses," Zammit said.

Marijuana is the most frequently used illegal substance in many countries,
including the United Kingdom and the United States. About 20 percent of
young adults report using it at least once a week, according to government
statistics.

Zammit and colleagues from the University of Bristol, Imperial College and
Cambridge University examined 35 studies that tracked tens of thousands of
people for periods ranging from one year to 27 years to examine the effect
of marijuana on mental health.

They looked for psychotic illnesses as well as cognitive disorders including
delusions and hallucinations, bipolar disorder, depression, anxiety,
neuroses and suicidal tendencies.

They found that people who used marijuana had roughly a 40 percent higher
chance of developing a psychotic disorder later in life. The overall risk
remains very low.

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For example, Zammit said the risk of developing schizophrenia for most
people is less than 1 percent. The prevalence of schizophrenia is believed
to be about five in 1,000 people. But because of the drug's wide popularity,
the researchers estimate that about 800 new cases of psychosis could be
prevented by reducing marijuana use.

The scientists found a more disturbing outlook for "heavy users" of pot,
those who used it daily or weekly: Their risk for psychosis jumped to a
range of 50 percent to 200 percent.

One doctor noted that people with a history of mental illness in their
families could be at higher risk. For them, marijuana use "could unmask the
underlying schizophrenia," said Dr. Deepak Cyril D'Souza, an associate
professor of psychiatry at Yale University, who was not involved in the
study.

Dr. Wilson Compton, a senior scientist at the National Institute on Drug
Abuse in Washington, called the study persuasive.

"The strongest case is that there are consistencies across all of the
studies," and that the link was seen only with psychoses -- not anxiety,
depression or other mental health problems, he said.

Scientists cannot rule out that pre-existing conditions could have led to
both marijuana use and later psychoses, he added.

Scientists think it is biologically possible that marijuana could cause
psychoses because it interrupts important neurotransmitters such as
dopamine. That can interfere with the brain's communication systems.

Some experts say governments should now work to dispel the misconception
that marijuana is a benign drug.

"We've reached the end of the road with these kinds of studies," said Dr.
Robin Murray of King's College, who had no role in the Lancet study.
"Experts are now agreed on the connection between cannabis and psychoses.
What we need now is for 14-year-olds to know it."

In the U.K., the government will soon reconsider how marijuana should be
classified in its hierarchy of drugs. In 2004, it was downgraded and
penalties for possession were reduced. Many expect marijuana will be bumped
up to a class "B" category, with offenses likely to lead to arrests or
longer jail sentences.

Two of the authors of the study were invited experts on the Advisory Council
on the Misuse of Drugs Cannabis Review in 2005. Several authors reported
being paid to attend drug company-sponsored meetings related to marijuana,
and one received consulting fees from companies that make antipsychotic
medications. E-mail to a friend

Copyright 2007 The Associated Press. All rights reserved.This material may
not be published, broadcast, rewritten,

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