Hormones Chronic marijuana use has not been found to alter testosterone or other sex hormone levels, despite the conclusions of Dr. R.C. Kolodony's 1974 study. Seven similar studies have been performed since then, the most recent by a Dr. Robert Block at the University of Iowa, and none have reproduced Kolodony's results. In contrast, heavy alcohol use is known to lower these same testosterone levels. Reproductive Damage No trustworthy study has ever shown that marijuana use damages the reproductive system, or causes chromosome breakage. Dr. Gabriel Nahas reached the opposite conclusion in his experiments performed in the early 1980s, but did so in part using the in vitro (i.e., in test tubes and petrii dishes) cells of rhesus monkeys. His rather unjustified claim that these changes would also occur in human bodies in vivo (in the body) was criticized by his colleagues and, in 1983, he renounced his own results. Studies of actual human populations have failed to demonstrate that marijuana adversely affects the reproductive system. Wu et al. found in 1988 a correlation between marijuana use and low sperm counts in human males. But this is misleading because (1) a decrease in sperm count has not been shown to have a negative effect on fertility, and (2) the sperm count returned to normal levels after marijuana use had ceased. Claims that marijuana use may impair hormone production, menstrual cycles, or fertility in females are both unproven and unfounded. The Immune System Studies in which lab rats were injected with extremely large quantities of THC have found that marijuana (in such unrealistically huge quantities) does have an "immunosuppressive effect" in those lab rats, in that it temporarily shuts off certain cells in the liver called lymphocytes and macrophages. These macrophages are useful in fighting off bacterial, not viral, infections. But this is only for the duration of intoxication. There also exists some evidence that marijuana metabolites stay in the lungs for up to seven months after smoking has ceased, possibly affecting the immune system of the lungs (but not by turning the cells off). This said, doctors and researchers are still not sure that the immune system is actually negatively affected in realistic situations since there are no numbers to support the idea. In fact, three studies showed that THC may have actually stimulated the immune system in the people studied. Birth Defects Unlike alcohol, cocaine, and tobacco, studies show that there exists no evident link between prenatal use of marijuana and birth defects or fetal alcohol syndrome in humans. In fact, marijuana use during the third trimester has been found to have a positive impact on birthweight. It is known that Delta-9-THC does enter the placenta, so mothers are advised against consuming large quantities.