Marijuana users may smoke less than cigarette users, but each puff they take from a joint contains more toxins than the average cigarette, WebMD reported Dec. 17.

Researchers who compared marijuana smoke to tobacco smoke found that ammonia levels were 20 times higher in the marijuana smoke, and that hydrogen cyanide and nitrogen-related chemicals also were more prevalent in the marijuana smoke. The latter likely came from fertilizer used to grow the marijuana, which came from Canada.

Unlike tobacco, however, marijuana smoke does not contain the addictive drug nicotine.

The study appears in the online edition of the journal Chemical Research in Toxicology.