The researchers burned 30 marijuana cigarettes and 30 tobacco cigarettes on a machine in their lab, measuring levels of chemicals in the smoke.
Ammonia levels were up to 20 times higher in marijuana smoke than in tobacco smoke. Levels of hydrogen cyanide and nitrogen-related chemicals were three to five times higher in marijuana smoke than in tobacco smoke.
The nitrogen-based fertilizer used on the marijuana plants -- which all came from the same batch of Canadian pot plants -- may have affected the results. The temperatures used to burn the cigarettes may also have been a factor.
Marijuana smoke and tobacco smoke shared many of the same chemicals. But the two types of smoke weren't identical.
For instance, marijuana doesn't contain nicotine. And tobacco doesn't contain cannabinoids, which include THC, marijuana's active ingredient
Tobacco has long been linked to cancer and other health problems. Marijuana smoke hasn't been tied to cancer in the past, note the researchers, who included David Moir, PhD, of the SafeEnvironments Programme in Kitchener, Ontario.
Moir and colleagues report their findings in the advance online edition of Chemical Research in Toxicology.
They promise to compare the toxicity of marijuana smoke and tobacco smoke in animals in another study.