macrophages, are primarily responsible for protecting the lungs against infectious microbes, harmful substances, and tumor cells. Compared with nonsmokers, habitual marijuana smokers in a large study were found to have twice as many alveolar macrophages, a sign that their lungs were fighting infection or invasion. People who smoked both marijuana and tobacco had four times as many of the cells as nonsmokers.9

Marijuana smoking was also found to reduce the ability of alveolar macrophages to destroy disease-causing fungus and bacteria as well as tumor cells. Moreover, marijuana smoking appears to depress macrophages ' ability to produce cytokines—hormonelike chemicals that help coordinate the immune response.10 Taken as a whole, these findings indicate that smoking marijuana could have dangerous consequences for patients with compromised immune systems, including people with AIDS and cancer—particularly those who are receiving immunosuppressive chemotherapy—as well as organ transplant recipients.

Exposure to cannabinoids can also affect the cardiovascular system. 11 Although these effects tend to be shortlived, they are far easier to measure than the impact of cannabinoids on the immune system. Both smoked marijuana and THC have been shown to raise heart rate, from 20 to 100 percent above normal in some cases. Oral THC (as well as smoked marijuana) can also exaggerate the drop in blood pressure that occurs when a person rises to standing after lying down, sometimes so much so that the person faints. This reaction rarely occurs after two to three days of repeated exposure to THC or marijuana extract, and it poses little risk for young healthy people. It could, however, present a serious problem for older patients or for people at risk for heart attack or stroke. As chronic marijuana users who began taking the drug during the 1960s approach the age at which cardiovascular disease becomes common, the impact of marijuana use on circulatory health should become clearer. In the meantime, people at risk for cardiovascular disease would be wise to avoid marijuana and THC.

In addition to effects on the immune and cardiovascular systems, researchers have considered the impact of cannabinoids on reproduction. 12 A series of reports involving experimental animals injected with THC indicate that it inhibits several different repro-

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