Men who smoke marijuana may have a better chance of having a baby with their female partner, compared with those who don't use the drug, a surprising new study suggests.
The study, which was published Aug. 14 in the journal Human Reproduction, involved several hundred couples undergoing fertility treatment with in vitro fertilization (IVF). The researchers found that women who reported currently using marijuana had a higher likelihood of pregnancy loss, compared with women who didn't use marijuana.
In contrast, couples whose male partner said he currently used marijuana had better chances of having a child, compared with couples whose male partner didn't currently use marijuana.
This finding was unexpected, according to the authors, from the Harvard T.H. Chan School of Public Health in Boston. The researchers had hypothesized that marijuana smoking wouldn't be related to fertility outcomes in either men or women, as has been the case in previous studies.
But the new result agrees with findings in an earlier study from the same group of researchers. In that study, men who reported ever having smoked marijuana had higher sperm counts, on average, than those who had never used the drug.
Still, the new findings don't mean that men should start smoking marijuana to boost their fertility. Only a small number of participants said they smoked marijuana around the time of their fertility treatments, which reduces the strength of the results. At most, they suggest that marijuana may not have a harmful effect on men's fertility, the authors said. On the other hand, the researchers don't think their findings should be taken as evidence that marijuana has a beneficial effect for men undergoing fertility treatment.
There is an urgent need for "additional research to clarify the role of marijuana use on human reproduction and on the offspring's health," the authors concluded.
Despite the growing use and legalization of marijuana around the world, scientists know little about how the drug impacts fertility. And few studies have included both men and women.
In the new study, the researchers analyzed information from 200 couples who underwent fertility treatment at Massachusetts General Hospital between 2005 and 2017. The researchers also included data from an additional 220 women who underwent fertility treatment, but did not have a partner in the study.
Participants were asked whether they were currently using marijuana, had used the drug in the past or had never used it.
Overall, 44% of the women and 61% of the men in the study reported they had smoked marijuana at some point in their lives. But just 12 women (3%) and 23 men (12%) in the study said they were currently using marjuana.
Among the small number of women who said they currently smoked marijuana and became pregnant during the study, more than 50% experienced a pregnancy loss, compared with just 26% of the women who were past marijuana users or who had never used the drug.
This finding suggests that marijuana use among women "may be related to worse infertility treatment outcomes," the authors said. But they caution that since very few women in the study were current marijuana users, it's possible that this finding was due to chance.
On the other hand, among couples whose male partner was a current marijuana user, 48% eventually had a live birth, compared with just 29% of couples whose male partner was a past marijuana user or who had never used it. The link held even after the researchers took into account some factors that could affect fertility, including the participants' age, ethnicity, body mass index (BMI), tobacco smoking history, coffee intake, alcohol use and cocaine use.
More and more patients are asking about the reproductive effects of marijuana, but doctors have had few studies to share when advising patients.
"At least weekly, I have patients asking me about the effects of marijauna on male fertility," said Dr. Neel Parekh, a urologist specializing in male fertility and men's health at the Cleveland Clinic's Glickman Urological & Kidney Institute. "There just isn't a great answer we can give them yet."
In this sense, the new study is "a step in the right direction," Parekh told Live Science.
However, the new study by itself isn't enough for doctors to recommend that men smoke marijuana prior to fertility treatment.
Parekh noted that, with only 23 men in the study reporting current use of marijuana, "It's hard to make that big of a statement saying marijuana is going to improve success rates" with fertility treatment.
But Parekh agreed with the authors that, rather than showing a benefit per se, the study suggests that smoking marijuana may not hurt the chances of success with fertility treatment when the male partner uses it.
The study authors note that their work included couples undergoing fertility treatment, and so the findings may not apply to couples trying to conceive without medical assistance. Indeed, Parekh noted that some forms of IVF use only a single sperm to fertilize an egg, and so with these treatments, a man's sperm count isn't usually a big deal. But when couples are trying to conceive naturally, sperm count matters more.
In addition, the new study only asked about marijuana smoking and not other forms of marijuana use.
More robust studies are now needed to look at this issue, said Parekh, and he expects to see more research in this area in the coming years.