Smoking a single marijuana cigarette may be as damaging to a person's health as smoking five tobacco cigarettes, according to a new study that supports a growing body of research suggesting that the drug may be more harmful than previously suspected.

The study, by researchers at the University of California at Los Angeles School of Medicine, found that marijuana smokers absorbed into the bloodstream nearly five times as much carbon monoxide and inhaled into their lungs three times as much tar as did tobacco smokers.

Noting that carbon monoxide has been linked to coronary artery disease and that tar has been associated with cancer, the researchers warned that smokers of a few marijuana cigarettes daily should be concerned about long-term damage.

"The bottom line is that smokers of just a couple of joints a day cannot lull themselves into a false sense of security that they will escape the pulmonary consequences of smoking just because they're smoking so few joints," said Dr. Donald P. Tashkin of UCLA.

The study appeared in Thursday's issue of the New England Journal of Medicine.

Nevertheless, proponents of marijuana legalization Wednesday questioned the purpose of tobacco-marijuana comparisons and suggested that marijuana smokers should heed Tashkin's findings by using a water pipe as a filter.

"In the current climate, people are very skeptical about these reports about marijuana and health," said Jon Gettman of the National Organization for the Reform of Marijuana Laws, who said that he feared that critics would use such studies to justify current bans on marijuana.

Tashkin, however, said a water pipe would not protect against tar.