Wayne Hall, Nadia Solowij & Jim Lemon

High doses of THC probably disturb the male and female reproductive systems in animals. They reduce secretion of testosterone, and hence reducing sperm production, motility, and viability in males. It is uncertain whether these effects also occur in humans. Studies in humans have produced both positive and negative evidence of an effect of cannabinoids on testosterone, for reasons that are not well understood. Hollister has argued that the reductions in testosterone and sperm production observed in the positive studies are probably of "little consequence in adults", although he conceded that they could be of "major importance in the prepubertal male who may use cannabis." The possible effects of cannabis use on testosterone and spermatogenesis may be most relevant to males whose fertility is already impaired for other reasons, e.g. a low sperm count.
(Please note: This text has been taken from a scientific article. Some sentences have been changed to improve understandability.)
Hall W, Solowij N, Lemon J. The Health and Psychological Consequences of Cannabis Use. National Drug Strategy Monograph Series No. 25. Canberra: Australian Government Publishing Service, 1994.
 

Laura Murphy

In human males, cannabis smoking has been shown to decrease blood levels of the three hormones LH, FSH, and testosterone. Moreover, an increased incidence of low sperm count has been reported in men who were heavy marijuana smokers. Other studies did not find measurable differences in men who were light or heavy marijuana users. Acute THC treatment produces a consistent and significant dose- and time-related decrease in LH and testosterone levels in male rodents. In the male rhesus monkey, an acute dose of THC produced a 65% reduction in blood testosterone levels by 60 min of treatment that lasted for approximately 24 hr.
(Please note: This text has been taken from a scientific text. Some sentences have been changed to improve understandability.)
Murphy L. Hormonal system and reproduction. In: Grotenhermen F, Russo E, eds. Grotenhermen, F., Russo, E. (eds.): Cannabis and cannabinoids. Pharmacology, toxicology, and therapeutic potential. Haworth Press, Binghamton/New York 2001, in press.
 

Lynn Zimmer & John Morgan

By giving large doses of THC to animals, researchers have produced appreciable effects on sex hormone levels. However, the effects vary from one study to another, depending on the dose and timing of administration. When effects occur, they are temporary. (...) In neither male nor female animals have researchers produced permanent harm to reproductive function from either acute or chronic marijuana administration. (...) There is no convincing evidence of infertility related to marijuana consumption in humans. There are no epidemiological studies showing that men who use marijuana have higher rates of infertility than men who do not. Nor is there evidence of diminished reproductive capacity among men in countries where marijuana use is common. It is possible that marijuana could cause infertility in men who already have low sperm counts, However, it is likely that regular marijuana users develop tolerance to marijuana's hormonal effects. (...) Marijuana has neither a masculinizing effect in females nor a feminizing effects in males.
Zimmer L, Morgan JP. Marijuana Myths Marijuana Facts. A review of the scientific evidence. New York/San Francisco: The Lindesmith Center, 1997.
 

House of Lords

Animal experiments have shown that cannabinoids cause alterations in both male and female sexual hormones; but there is no evidence that cannabis adversely affects human fertility, or that it causes chromosomal or genetic damage.
House of Lords Select Committee on Science and Technology. Cannabis. The scientific and medical evidence. London: The Stationery Office, 1998.