Within a few minutes after inhaling marijuana smoke, an individual’s heart rate speeds up, the bronchial passages relax and become enlarged, and blood vessels in the eyes expand, making the eyes look red. The heart rate – normally 70 to 80 beats per minute – may increase by 20 to 50 beats per minute, or may even double in some cases. Taking other drugs with marijuana can amplify this effect.
Limited evidence suggests that a person’s risk of heart attack during the first hour after smoking marijuana is four times his or her usual risk. This observation could be partly explained by marijuana raising blood pressure (in some cases) and heart rate and reducing the blood’s capacity to carry oxygen. Such possibilities need to be examined more closely, particularly since current marijuana users include adults from the baby boomer generation, who may have other cardiovascular risks that may increase their vulnerability.
The smoke of marijuana, like that of tobacco, consists of a toxic mixture of gases and particulates, many of which are known to be harmful to the lungs. Someone who smokes marijuana regularly may have many of the same respiratory problems that tobacco smokers do, such as daily cough and phlegm production, more frequent acute chest illnesses, and a greater risk of lung infections. Even infrequent marijuana use can cause burning and stinging of the mouth and throat, often accompanied by a heavy cough. One study found that extra sick days used by frequent marijuana smokers were often because of respiratory illness.6
In addition, marijuana has the potential to promote cancer of the lungs and other parts of the respiratory tract because it contains irritants and carcinogens – up to 70 percent more than tobacco smoke. It also induces high levels of an enzyme that converts certain hydrocarbons into their cancer-causing form, which could accelerate the changes that ultimately produce malignant cells. And since marijuana smokers generally inhale more deeply and hold their breath longer than tobacco smokers, the lungs are exposed longer to carcinogenic smoke. However, while several lines of evidence have suggested that marijuana use may lead to lung cancer, the supporting evidence is inconclusive.8 The presence of an unidentified active ingredient in cannabis smoke having protective properties – if corroborated and properly characterized – could help explain the inconsistencies and modest findings.
A significant body of research demonstrates negative effects of THC on the function of various immune cells, both in vitro in cells and in vivo with test animals. However, no studies to date connect marijuana’s suspected immune system suppression with greater incidence of infections or immune disorders in humans. One short (3-week) study found marijuana smoking to be associated with a few statistically significant negative effects on the immune function of AIDS patients; a second small study of college students also suggested the possibility of marijuana having adverse effects on immune system functioning. Thus, the combined evidence from animal studies plus the limited human data available seem to warrant additional research on the impact of marijuana on the immune system.
6Polen, M.R.; Sidney, S.; Tekawa, I.S.; Sadler, M.; and Friedman, G.D. Health care use by frequent marijuana smokers who do not smoke tobacco. West J Med 158(6): 596-601, 1993.
8Hashibe, M.; Morgenstern, H.; Cui, Y.; Tashkin, D.P.; Zhang, Z.F.; Cozen, W.; Mack, T.M.; and Greenland, S. Marijuana use and the risk of lung and upper aerodigestive tract cancers: Results of a population-based case-control study. Cancer Epidemiol Biomarkers Prev 15(10): 1829-1834, 2006.