Fertility in both female and male humans is not significantly impaired by natural cannabis or isolated cannabinoid compounds. Animal studies have found that extremely high doses of synthetic THC can mildly inhibit ovulation and testosterone production, but that inhibition dissipated with continued administration. Some human studies show mild depression of hormone levels within normal basal ranges following administration of a large dose (2.0-2.8% THC cigarettes) of cannabis, but these levels also returned to normal upon continued exposure. According to the Australian government’s recent review of the available data:
Even if there are such effects of cannabis on male reproductive functioning, their clinical significance in humans is uncertain (Institute of Medicine, 1982) since testosterone levels in the studies which have found effects have generally remained within the normal range (Hollister, 1986)… In the absence of any other human evidence, both Bloch (1983) and the Institute of Medicine (1982) argued on the basis of animal literature that cannabis use probably had an inhibitory effect on female reproductive function which was similar to that which occurs in males.
Medical authorities Zimmer and Morgan offer real-world evidence and rational insight:
There are no epidemiological studies showing that men who use marijuana have higher rates of infertility than men who do not. Not 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 develops tolerance to marijuana’s hormonal effects.
Related sections: Immune Responses, Obstetrics, Testosterone, Tolerance.
 Cone, Johnson, Moore, and Roache, “Acute effects of smoking marijuana on hormones, subjective effects and performance in male human subjects.” Pharmacology and Biochemical Behavior, Vol. 24, No. 6, pp. 1749-1754, June 1986
 “The health and psychological consequences of cannabis use.” Chapter 6, National Drug Strategy Monograph No. 25, Australia
 Zimmer and Morgan, Marijuana Myths: Marijuana Facts. New York: The Lindesmith Center, 1997