Men may be doubling their partner’s risk of miscarriage with a single controversial habit

Your spirits won’t be high after reading this.

A growing body of research suggests a controversial, increasingly common habit in the US may be quietly damaging men’s sperm — and doubling their partner’s risk of miscarriage.

“It is one of the most moveable factors right now in this country for improving fertility outcomes,” Dr. Natalie Crawford, a reproductive endocrinologist, said on a recent episode of the “Huberman Lab” podcast.

Across the nation, 24 states and the District of Columbia have legalized the recreational use of marijuana for adults, with 18 others allowing it for medical use.

That wave of legalization over the past two decades has gone hand in hand with a surge in cannabis use. Today, roughly 18 million Americans report using it daily — surpassing, for the first time in 2022, those who say they drink alcohol at the same frequency.

But research suggests even once-a-week tokers may be hurting their chances of producing healthy offspring.

In a 2019 study, researchers at Boston University analyzed data from more than 1,400 heterosexual couples trying to conceive across the US and Canada.

The participants were surveyed every two months on a range of behaviors, including cannabis use, leading up to conception and again at various stages of pregnancy.

The researchers found that men who used marijuana at least once a week faced roughly twice the chance of their partner’s pregnancy ending in miscarriage compared to men who did not use cannabis.

That pattern held even after the team accounted for whether the woman used cannabis herself.

Among the women who miscarried, pregnancy loss typically occurred at around six weeks.

After eight weeks of pregnancy, the link between male marijuana use and miscarriage dropped off significantly.

The findings support the researchers’ theory that marijuana use in men may affect the earliest stages of pregnancy by impacting sperm quality.

“There are [cannabinoid] receptors in human testicular tissue and sperm, and when a man uses marijuana, THC binds to these receptors,” Alyssa Harlow, a Boston University School of Public Health researcher and lead author of the study, told the Brink at the time.

Crawford, who was not involved in the study, says she has seen the effects firsthand, calling cannabis “hugely detrimental” to sperm production and quality.

“In the IVF lab, when I see embryos halt at that male developmental stage on day three, we’ll say, ‘This is a young couple. They’ve got no embryos, and we were expecting some,’” she told Andrew Huberman.

“When we go back, nine out of 10 times, he’s using cannabis that he previously denied.”

Other research has raised similar concerns about cannabis and sperm.

A 2019 review that looked at 48 studies on male fertility found that cannabis use was tied to lower sperm counts, reduced concentration and a higher rate of abnormally shaped sperm.

A year later, researchers studying 229 men in Jamaica found that even moderate cannabis use was linked to nearly 3.5 times the likelihood of producing misshapen sperm.

And in 2025, a study of 113 Jordanian men found sperm movement was significantly lower in cannabis users compared with tobacco smokers and nonsmokers.

Similar effects have also been seen in animal studies.

In 2020, researchers at Duke University found that male rats exposed to marijuana showed changes in their sperm before mating. Their offspring later developed abnormalities in brain regions involved in learning, memory, reward and mood.

But not all research points in the same direction.

For example, Harvard researchers found that men who had smoked marijuana at some point in their lives had significantly higher sperm concentrations compared with men who had never used the drug.

“These unexpected findings highlight how little we know about the reproductive health effects of marijuana, and in fact of the health effects of marijuana in general,” Jorge Chavarro, an associate professor of nutrition and epidemiology at Harvard’s T.H. Chan School of Public Health, said in a 2019 press release.

Part of the challenge, researchers say, is that despite shifting state laws, marijuana remains illegal under federal law — making large-scale research difficult.

The Trump administration has signaled plans to reschedule the drug under the Controlled Substances Act in an effort to make research easier. The timeline for that change remains unclear.

For now, Harlow says more research is needed before scientists can definitively say the impact cannabis use has on male fertility.

“We need more studies before we can make any concrete recommendations,” she said.

“For couples trying to get pregnant, they should try to live as healthy a lifestyle as possible and talk to their doctor about marijuana use.”

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