"Although the use of [marijuana] is not harmless, the current knowledge base does not support the assertion that it has any notable adverse public health impact in relation to mortality," Sidney concludes.

It's been feared that marijuana smoke, like tobacco smoke, causes cancer and heart disease . The evidence argues otherwise, writes Stephen Sidney, MD, associate director for research for Kaiser Permanente, Oakland, Calif., in the Sept 20 issue of The British Medical Journal .

No Marijuana Deaths in 2 Large Studies

Sidney points to two large studies. The first is from (where else?) California. A large HMO looked at 65,177 men and women age 15-49. Over 10 years, marijuana users died no sooner than nonusers.

The second study looked at 45,450 Swedish army conscripts. They were 18-20 years old when asked about marijuana use. Fifteen years later, the marijuana users were just as likely to remain alive as nonusers.

And since marijuana smoking can't kill outright -- there's no such thing as a fatal marijuana overdose -- short-term use isn't deadly. Long-term use can't be good for you. But Sidney notes that most marijuana smokers don't become long-term users.

One worry about marijuana smoke is that it is inhaled, and held, deep in the lungs. But the typical user smokes only one marijuana cigarette -- or less -- a day. Tobacco users often smoke 20 or more cigarettes daily. Moreover, tobacco contains nicotine, a highly addictive substance. Marijuana, Sidney concludes, is less likely to harm than tobacco.

A 2001 study suggested that marijuana smoking increases the risk of heart attack in the hour immediately after smoking. But this seems to be the case in no more than one-fifth of 1% of heart attacks -- a very rare risk indeed.