Marijuana smoke contains several of the same carcinogens as tobacco, but in higher concentrations, raising concerns that smoking marijuana may be a risk factor for tobacco-related cancers. Unfortunately, studies are difficult to conduct because some people may not be truthful about use of marijuana given that it is illegal. It is difficult to accurately assess how often it is used and to determine cumulative exposure. In addition, studies must consider that many individuals who use marijuana also smoke tobacco.
Marijuana cigarettes tend to be smoked without filters, to a smaller butt size and be inhaled more deeply. This leads to higher concentrations of smoke being drawn deeper into the respiratory tract than occurs during cigarette smoking, and is associated with 5 times greater absorption of carbon monoxide. In relation to cigarette smoking, marijuana smoking may involve the inhaling of 3 times the amount of tar and 30% more of this tar is retained in the lungs. Benzopyrene, a cancer-causing agent, is present in higher concentrations in marijuana than in cigarettes.
One recent study found that the risk of lung cancer from smoking one joint a day to be similar to the risk from smoking 1pack of cigarettes per day. When pathologists looked at the lungs of deceased marijuana smokers, they saw many abnormalities. The upper respiratory tract damage was similar to a pack-a-day cigarette smoker.
Marijuana smoking is also known to cause damage to lung tissue, resulting in asthma and chronic obstructive lung disease (COPD). Researchers caution that smoking marijuana may also decrease reproductive function and increase the risk of cancer of the mouth and tongue. It may also suppress the body's immune system and increase the risk of leukemia in children whose mothers smoke marijuana during pregnancy.
Some researchers have proposed conducting studies in countries where marijuana is legal, but this has yet to be done. The other factor is that, unlike cigarette smokers, most marijuana smokers do not smoke chronically or in the volume that cigarette smokers do, so focusing on the groups that are chronic marijuana users may be more helpful in studies. The bottom line is, researchers agree that there is most likely some cancer-causing effect to smoking marijuana, but they have yet to quantify this with clinical studies.