July 27 (Bloomberg) -- Smoking marijuana increases the risk of developing psychosis later in life by 40 percent, with heavy users having an even higher risk of mental problems, British researchers said.

The researchers analyzed 35 previous studies and concluded that using marijuana or cannabis raised the risk of hallucinating and being delusional later in life.

Marijuana is the most commonly used illegal drug in countries including the U.S. and the U.K., the authors, led by Glyn Lewis of the University of Bristol, said in the study, published today in The Lancet. Reducing the use of cannabis may help avoid 14 percent of psychoses in Britain, they said.

``We now know that there is a long-term risk associated with the use of cannabis,'' Merete Nordentoft, of the department of psychiatry at the Copenhagen University Hospital, said in an interview. Nordentoft, in a commentary accompanying the study, said there is a need to warn the public and establish treatments to help users.

The studies that were examined included patients with disorders such as psychosis, schizophrenia, delusions and hallucinations, the researchers said. A link to depression or obsessive compulsive disorder couldn't be as clearly established, they said. The research was funded by the U.K.'s Department of Health.

The risk of developing a mental illness is linked to the amount and frequency of marijuana consumed, with the danger for frequent users increasing as much as threefold. Still, an individual cannabis user's risk of developing a disorder is ``low,'' Lewis said in an interview.

Scientific Standard

Nordentoft said the study is the best to date, although like any meta-analysis it is only as strong as the weakest trial under review. Because a large-scale trial of an illegal drug isn't feasible, the results can't be confirmed by the highest scientific standard, a double-blind study.

The short-term effects of marijuana can include problems with memory and learning, distorted perception, difficulty in thinking and problem solving, loss of coordination and increased heart rate, according to the National Institutes of Health.

Earlier this month, the U.K. government said it may introduce stronger penalties for the possession of cannabis, which was effectively decriminalized in 2004. Whatever the U.K. decides, ``governments would do well to invest in sustained and effective campaigns on the risks to health of taking cannabis.''

Officials of the Marijuana Policy Project, which advocates the elimination of criminal penalties for cannabis use in the U.S., said that the study doesn't support prohibition as an effective response to perceived risks of marijuana. ``Marijuana use has actually dropped in Britain since the British ended most marijuana arrests in 2004,'' the Washington-based group's spokesman, Bruce Mirken, said in an e-mail.

In 2005, 24 percent of 16 to 24-year-olds in the U.K. admitted using the drug, down from 28 percent in 1998, according to the government's Home Office.

To contact the reporter on this story: Eva von Schaper in Munich at evonschaper@bloomberg.net.

Last Updated: July 27, 2007 03:20 EDT