Smoking marijuana more than once a week may lower men's sperm counts by about a third, according to a new study.
Researchers found that the men in the study who smoked marijuana more than once a week had sperm counts that were 29 percent lower, on average, than those who did not smoke marijuana, or used the drug less frequently.
The researchers thought that amount of reduction in sperm count "was quite a lot," said study author Tina Kold Jensen, of the University of Copenhagen in Denmark.
They also found that the sperm concentrations (which is measured slightly differently than sperm count) were 28 percent lower in the men who smoked marijuana more than once a week.
It is not clear why marijuana use may decrease a man's sperm count and concentration, but it may have something to do with how THC — marijuana's psychoactive ingredient — interacts with certain receptors in the testes, the researchers said.
In the study, the researchers asked 1,215 Danish men ages 18 to 28 whether they used marijuana and other drugs like amphetamine, ecstasy and cocaine within the past three months, and if so, how often. [11 Odd Facts About Marijuana]
The researchers also collect semen samples, to measure the men's sperm counts and concentrations.
About 45 percent of the men in the study said they had smoked marijuana in the past three months. About 10 percent of the study participants had used marijuana as well as recreational drugs during this time.
Moreover, in the men in the study who smoked marijuana and used other recreational drugs, the sperm counts were reduced by 55 percent, and the sperm concentrations were reduced by 52 percent, compared with men who hadn't used the drugs.
The researchers found that the men who had smoked marijuana in the past three months were also more likely to smoke cigarettes, and consumed more alcohol and caffeine than those who had not.
These lifestyle factors also could have affected the men's sperm levels, so the researchers took them into account, and the link between marijuana and lower sperm levels held. However, the study still does not prove that the drug caused the lower sperm counts, Jensen said.
"We cannot exclude the possibility that the men who used marijuana generally have an unhealthier lifestyle and health behavior, which may also affect their semen quality and hormone levels," the researchers said in the study.
"It is … important to note that marijuana users were distinct in several ways from nonusers," Dr. Michael L. Eisenberg of Stanford University School of Medicine, who was not involved in the new study, wrote in an editorial accompanying the study in the journal. And although the researchers adjusted for these differences, such as higher rates of cigarette smoking and alcohol consumption, they could have still confounded the potential effects of marijuana on sperm, he wrote.
Still, the new study "provides important information for patients and providers regarding the negative association between marijuana use and semen quality," Eisenberg wrote.
The study was published Aug. 16 in the American Journal of Epidemiology.
Sign up for the Live Science daily newsletter now
Get the world’s most fascinating discoveries delivered straight to your inbox.