Several interesting lines of research suggest that the cannabinoids play an important role in the function of sex hormones and sperm. This work has led to growing concerns about marijuana's impact on fertility and birth defects. Unfortunately, some enthusiastic efforts to prevent drug use have overstated the data, implying that consumption can lead to permanent infertility or drastic birth defects. (My junior high school health teacher suggested that the children of marijuana smokers would be born with one eye in the center of their foreheads.) Extremely large doses of cannabis could, in theory, decrease fertility in humans. Nevertheless, research has yet to show a definitive decrease in reproductive function in people who smoke marijuana. The drug also has not been linked to human birth defects.
Studies of primates show that THC can alter sex hormones. Large
doses of the drug can decrease hormones central to menstruation, including follicle-stimulating hormone, luteinizing hormone, and progesterone. These effects require an injected dose of 2.5 mg of THC per kilogram of body weight, a dose comparable to a 130-pound woman smoking 7 joints per day. Monkeys who received this dose for 18 consecutive days did not ovulate. Nevertheless, with chronic administration of the drug for up to a year, monkeys developed tolerance to this effect and began ovulating again. Comparable results appear for rats and rabbits. Thus, large doses can disrupt female sex hormones, but functions return to normal with time (Smith, Almirez, Scher, & Asch, 1984).
Research on marijuana's impact on fertility in human women offers mixed results. One study showed increased rates of marijuana use in women who are infertile. In the infertile group, 61% of the women had used marijuana. In the fertile group, only 53% had smoked cannabis. Nevertheless, infertile women did not use marijuana more often or for a longer period than those who were fertile. The use of cocaine had a much larger impact on fertility. Once the investigators controlled for cocaine consumption and other factors that contribute to infertility, the impact of cannabis decreased (Mueller, Daling, Weiss, & Moore, 1990). In contrast, a later study found that women who used marijuana regularly conceived more quickly than women who did not use the drug (Joesof, Beral, Aral, Rolfs, & Cramer, 1993). Perhaps these data say more about marijuana's alleged functioning as an aphrodisiac than anything about a direct impact on fertility.
Animal and human research also reveals that THC may interfere with the production of sperm. Extensive studies of sea urchin sperm reveal that cannabinoids decrease their capacity to fertilize an egg (Schuel et al., 1999). In addition, THC lowers the motility of bull sperm in a petri dish (Shahar & Bino, 1974). THC treatments of 5 mg per kilogram of body weight (18 joints per day for a 160-pound man) created twice as many abnormal sperm as usual in mice (Zimmerman, Zimmerman, & Raj, 1979). Anyone intending to breed mice, cows, or sea urchins should keep them away from large doses of cannabinoids. In human research, men who smoked an average of 8 joints per day for one month showed significant drops in sperm count and motility (Hembree, Nahas, Zeidenberg, & Huang, 1979). Nevertheless, their sperm count and functions did not fall to abnormal levels and returned to normal after the study ended. Despite all this work, no studies show an actual decrease in fertility for men who use cannabis.
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