"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.”

Scientists at Oregon Health & Science University (OHSU) say that non-human primates administered cannabis edibles daily experienced a significant adverse effect their fertility and reproductive health.

“Our analysis of the collected samples found that THC use was associated with significant adverse impacts to the animals’ reproductive hormones,” reports Dr. Jamie Lo, the study’s senior author and an associate professor of obstetrics and gynecology with the Oregon National Primate Research Center at OHSU.

These impacts included decreased testosterone levels and severe testicular shrinkage, Lo notes in an OHSU statement, adding that researchers observed a more than 50 per cent reduction in some subjects.

But that wasn’t everything. Lo says adverse impacts on reproductive health “appeared to worsen as the THC dose was increased, suggesting a possible dose-dependent effect.”

Published this week in Fertility and Sterility, that was the question investigators sought to answer for non-human primates: Is there is a dose-dependent effect of THC exposure on testes and reproductive health?

Researchers monitored the reproductive systems of healthy adult male rhesus macaques, aged eight to 10 years, who were administered “daily edible THC at medically and recreationally relevant doses,” notes the study, which is supported by National Institutes of Health (NIH) grants and was reviewed and approved by the OHSU Institutional Animal Care and Use Committee.

The rhesus macaques were all of reproductive age, had a history of proven paternity and had no prior cannabis exposure. As well, the THC dose “was based on published medical marijuana acclimation recommendations for humans.”

Over seven months, dose amounts were increased every 70 days, in line with the animals’ sperm development cycle. Increases were made until the dose “reached the equivalent of a heavy medical marijuana dose in humans,” OHSU reports.

“The study is the first to assess the impacts of substance use on testicular function via a mode of delivery, and dosage, that reflects current human consumption,” the university statement notes.

Additionally, study authors write that there were “significant dose-response decreases in mean total testosterone level,” but “no statistically significant changes in semen parameters.”

Overall, study authors point out that chronic THC exposure resulted in a significant dose-response “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.”

Admitting the need for more research, lead author Dr. Jason Hedges, an associate professor of urology in OHSU’s School of Medicine, suggests “these early findings are concerning from a clinical standpoint.”

Hedges notes “as the prevalence of edible marijuana use continues to increase in the U.S. and worldwide — particularly in males of prime reproductive age — even moderate doses could have a profound impact on fertility outcomes.”

Indeed, he cautions, “it is possible that THC could cause lasting impacts that may alter family planning later in life.”

Results of an earlier study of female rhesus macaques and the impact of THC on reproductive hormones, also headed by Low, are in line with the latest research.

An NIH paper published last year reported that women who use cannabis could have more difficulty conceiving a child than women who abstain from the drug.

“Women who said they used cannabis products — marijuana or hashish — in the weeks before pregnancy, or who had positive urine tests for cannabis use, were around 40 per cent less likely to conceive per monthly cycle than women who did not use cannabis,” an institute statement at the time noted.

Another study, this time focusing on males and published in 2019, found “cannabis may negatively impact male fertility.” The authors advised that “studies are needed to validate that robust findings in animal models will carry over into human experience.”

Per Urology Associates, “men who use marijuana more than once a week have nearly a 30 per cent reduction in sperm count and sperm concentration, and this has been shown to last five to six weeks even after they stop using marijuana.”