Physiological Harms of Cannabis

Marijuana is not associated with any increase of mortality. Most physiological harms that are attributed to marijuana are actually from the act of smoking, or inhaling smoke. They are very similar to tobacco smoke.

THC and the Immune System

THC both increases and decreases different types of immune system cells, but after much research there is no proof that it either weakens or strengthens the immune system.

Marijuana Smoke

Marijuana smoke is very similar to tobacco smoke, and tobacco smoke has been found to cause lung diseases like cancer and emphysema. Marijuana users typically do not smoke anywhere near the volume of marijuana as tobacco smokers smoke tobacco. One joint usually has about half the smokable substance as one tobacco cigarette.

But because marijuana is not usually filtered and users tend to hold the medical marijuana in their lungs for longer periods of time, up to four times as much tar can be deposited in the lungs per ounce of smoked marijuana as an ounce of tobacco. There is an additive effect of smoking both tobacco and marijuana.

Chronic Bronchitis

Smoking marijuana increases the users risk of respiratory illness. Interestingly people that have been smoking marijuana 10 years or more are usually not at a greater risk of having respiratory problems, but those that have been smoking for less than 10 years are. This is most likely because the people that do have respiratory problems discontinue smoking marijuana, and those that don’t quit usually haven’t had many problems. From a random sample 15-20% of people who smoke marijuana but not tobacco will report chronic bronchitis, and 20-25% of those that smoke tobacco but not marijuana will report chronic bronchitis. Those that smoke both will be at a higher risk.

Bronchial Tissue Changes

Heavy marijuana smokers will have changes in the lining of the respiratory tract. There will be more redness and swelling of the airway tissues and increased mucus secretion. Smokers of marijuana and tobacco have significantly more cellular and molecular abnormalities in the bronchial epithelium cells. There is an association with those abnormalities and increased risk of cancer. A marijuana only smoker that smokes about 21 marijuana cigarettes a week has about the same amount of abnormalities as a tobacco only smoker that smokes about 25 tobacco cigarettes a day. Once again those that smoke both will have an additive effect.


To date, there is no conclusive evidence that marijuana causes cancer in humans. However, marijuana has many of the same carcinogens as tobacco. The problem with finding a correlation with marijuana and cancer is that respiratory cancer requires a long-term exposure to smoke. Marijuana usage has only become popular since the late 1960’s, and most people only smoke marijuana when they are younger. As some habitual marijuana smokers get older and have been smoking marijuana for several decades there could very well be proof that marijuana causes cancer, but as of today there is no conclusive evidence that this is the case.