Like tobacco smoke, marijuana smoke contains cancer-causing chemicals. There are 33 cancer-causing chemicals contained in marijuana. Marijuana smoke also deposits tar into the lungs. In fact, when equal amounts of marijuana and tobacco are smoked, marijuana deposits four times as much tar into the lungs. This is because marijuana joints are un-filtered and often more deeply inhaled than cigarettes.
Marijuana smoke is also an irritant to the lungs, and frequent marijuana smokers can have many of the same respiratory problems experienced by people who smoke tobacco. These include coughing and phlegm production on most days, wheezing, bronchitis, and greater risk of lung infection.