(Reuters Health) - Men who have smoked marijuana may be more fertile than those who’ve never touched a joint, new data suggest, surprising researchers who had expected the opposite.

Men who smoked more than two joints in their lifetime had a significantly higher sperm concentration than those who had never smoked marijuana: on average, 62.7 million sperm per milliliter of semen versus 45.4 million/mL, Harvard researchers report in the journal Human Reproduction.

The pattern was similar when researchers looked at the men’s sperm count. The count was higher, on average, in men who had ever smoked marijuana, with no significant differences between those who had only smoked weed in the past and those who were currently using it.

Observational studies like this one can’t prove cause and effect - and the lead author of the study was quick to warn against assuming that marijuana actually caused the men in the study to have healthier sperm.

“These findings do not mean that using marijuana will increase sperm counts,” lead author Feiby Nassan emphasized in an email. “These results should not be interpreted as a reason to smoke marijuana.”

Nassan’s team studied 1,143 samples of semen obtained from 662 men treated at the fertility center at Boston’s Massachusetts General Hospital between 2000 and 2017. They also studied hundreds of blood samples from these men to determine levels of reproductive hormones. The men were asked to report past and current marijuana use in a questionnaire.

Weed smokers also performed better on another metric used to measure fertility, the level of follicle stimulating hormone found in the blood.

And they were also less likely to have poor sperm motility, which is the ability of sperm to swim toward an egg.

However, when researchers studied sperm DNA integrity - which assesses breakages in strands of sperm DNA - and levels of other hormones, there were no significant differences between marijuana smokers and non-smokers.

The study is “a real surprise,” said reproductive endocrinologist Dr. Channa Jayasena of Imperial College London, who was not involved in the study. “We obviously need to be very careful about members of the public jumping on to this.”

The findings are thought-provoking, he said. “This is a significant, large study and it’s been done to a high quality so the results can’t be ignored . . . It would be interesting to explore this further.”

Independent expert Dr. Michael Eisenberg, director of male reproductive medicine and surgery at Stanford University in California, believes it is too soon to make clinical recommendations about marijuana use based on the study findings.

Cannabinoid receptors, which are cells that play a role in marijuana’s effect in the body, have also been shown to be important in male fertility, he said, adding, “It is possible they are activated by marijuana use to the benefit of (sperm development).”

“Given the wave of legalization and the common use, this is certainly something that warrants further study,” he said.

While marijuana use is illegal under U.S. federal law, several states have passed laws allowing its recreational and medical use.

In the past, most research on the drug’s effects on male fertility have focused on substance abusers, linking cannabis-smoking to lower sperm counts, the researchers note in their paper.

“Based on the preponderance of previous findings, we hypothesized that marijuana smoking would be associated with worse semen quality and lower serum testosterone,” they said.

The results, however, proved contrary to this hypothesis, even after the authors conducted sensitivity analyses and considered different metrics of marijuana smoking.

Nassan points to two possible explanations. One is that low-level marijuana use might benefit sperm production but the effect might be reversed with heavier use. Equally possible, she said, is that men with higher testosterone levels are more likely to engage in risk-seeking behaviors, including marijuana smoking.

“The study is a great opportunity to spark interest in investigating the health effects of marijuana,” she said.

SOURCE: bit.ly/2Ns37yW Human Reproduction, online February 6, 2019.