In a finding that challenges the increasingly popular belief that smoking marijuana is less harmful to health than smoking tobacco, researchers in Canada are reporting that smoking marijuana, like smoking tobacco, has toxic effects on cells.

Rebecca Maertens and colleagues note that people often view marijuana as a “natural” product and less harmful than tobacco. As public attitudes toward marijuana change and legal restrictions ease in some countries, use of marijuana is increasing.
Scientists know that marijuana smoke has adverse effects on the lungs. However, there is little knowledge about marijuana’s potential to cause lung cancer due to the difficulty in identifying and studying people who have smoked only marijuana.
The new study begins to address that question by comparing marijuana smoke vs. tobacco smoke in terms of toxicity to cells and to DNA. Scientists exposed cultured animal cells and bacteria to condensed smoke samples from both marijuana and tobacco. There were distinct differences in the degree and type of toxicity elicited by marijuana and cigarette smoke.
Marijuana smoke caused significantly more damage to cells and DNA than tobacco smoke, the researchers note. However, tobacco smoke caused chromosome damage while marijuana did not.

Source: The Genotoxicity of Mainstream and Sidestream Marijuana and Tobacco Smoke Condensates. Chemical Research in Toxicology, Online July 17, 2009 DOI: 10.1021/tx9000286