A new animal study reveals that heavy use of cannabis can contribute to lower semen volume, decreased hormone production and testicular atrophy.

Researchers tracked for seven months the effects of a cohort of male rhesus macaques eating a daily cookie before breakfast containing tetrahydrocannabinol, or THC, which is the chemical responsible for marijuana’s psychoactive effects. The macaque subjects had previously fathered offspring.

As the dose of THC was gradually increased over the study period, researchers tracked the effect of exposure on the subjects’ testes and reproductive health.

Researchers collected blood and semen samples from the research subjects and performed scrotal ultrasounds to measure testicular volume of the macaques at the Oregon National Primate Research Center.

According to the study, published in the journal Fertility and Sterility, the macaques’ testicular volume decreased by 58% by the end of the study period. 

A change in hormone levels was also observed, including a decrease in testosterone. Semen volume was also reduced but no statistically significant drop in sperm count was observed.

The study concluded: “In rhesus macaques, chronic exposure to THC resulted in significant dose-response testicular atrophy, increased serum gonadotropin levels, and decreased serum sex steroids, suggestive of primary testicular failure. Further studies are needed to determine if reversal of these observed adverse effects would occur if THC was discontinued and for validation of the findings in a human cohort.”

Dr. Marcelle Cedars, president of the American Society for Reproductive Medicine, said in a statement as growing numbers of jurisdictions legalize cannabis for medical and recreational use, it is vital that medical science understand its impact on health, including reproduction.

“This study raises some important concerns, ones that need to be more fully explored,” Cedars said.

In approximately 40% of infertile of human couples, the male partner is either the sole cause or a contributing cause of infertility, according to the society’s website.

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