Men, if you’re thinking about starting a family, you may want to cut back on the marijuana. According to a recent study published in the American Journal of Epidemiology, smoking pot more than once a week can lower sperm count by a third, and if combined with other recreational drugs, including cocaine and ecstasy, it can fall to more than half.

The effects of marijuana on fertility have become of particular interest for researchers due to its increasing popularity among young males. Men who smoke marijuana do get women pregnant, but some men are more fertile than others, or are more fertile at different stages of their life. However, men who are considered to be borderline-infertile may risk infertility with smoking pot.

A 2003 study from the University of Buffalo found smoking marijuana can make sperm less fertile, even if the woman is the one who smokes it. When women smoke marijuana, nicotine, or other drugs, their reproductive fluids, or vaginal secretions will contain these drugs. For example, in marijuana, tetrahydrocannabinol (THC) — the drug’s psychoactive ingredient — is found in the vagina, the oviduct, and the uterus when a woman smokes. Even if a man hasn’t smoked but the woman has, his sperm will enter her body and reach the THC in her system, which is what causes the THC to alter his sperm.

But how does THC interact with sperm when a man smokes pot?

Tina Kold Jensen, lead author of the study of the University of Copenhagen and her colleagues, analyzed over 1,200 Danish men ages 18 to 28 who were recruited between 2008 and 2012, after attending a compulsory medical examination to determine their fitness service. The participants were asked whether they used marijuana and other drugs like amphetamine, ecstasy, and cocaine within the past three months, and if so, how often. The men provided semen samples, had a blood sample taken, and underwent a physical examination.

If the men admitted to taking drugs, they were asked how often on a scale from no use, to once a week, and more than once a week. The researchers then collected the semen samples to measure their sperm counts and concentrations.

The findings revealed about 45 percent of the participants reported smoking pot in the past three months. About 10 percent had used marijuana as well as recreational drugs during this time. Those who admitted to regularly smoking pot had a sperm count that was 29 percent lower than those who reported lighter use or didn’t smoke at all. Meanwhile, men who used other recreational drugs alongside marijuana had sperm counts reduced by 55 percent and sperm concentrations reduced by 52 percent.

Pot smokers were also found to have higher levels of testosterone within the same range as cigarette smokers. It’s no surprise to find similarities across these two drugs, since smoking cigarettes is a well-known lifestyle factor that can affect men’s sperm levels. A 2010 study published in the journal Human Reproduction found cigarettes damage sperm, making them less likely to fertilize eggs and less likely to have the embryos survive. This is because sperm begins to lose its ability to fight off free radicals in the seminal fluid once it's been exposed to cigarette smoke. Not only do the sperm cells become more sensitive to oxidative stress, but there is an increase in the amount of free radicals present in the seminal fluid.

Even after this was taken into account, the Danish researchers still found a link between marijuana and lower sperm levels, although they’re not clear on why the drug damages a man’s sperm count. Researchers speculate it could be linked to how THC interacts with certain receptors in the testes.

"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," wrote the researchers in the study.

Men, think before you huff and pass, if you want to be a dad some day.

Sources: Gundersen TD, Jorgensen N, Andersson AM et al. Association Between Use of Marijuana and Male Reproductive Hormones and Semen Quality: A Study Among 1,215 Healthy Young Me. Am. J. Epidemiol. 2015.

Hammadeh ME, Hamad MF, Montenarh M et al. Protamine contents and P1/P2 ratio in human spermatozoa from smokers and non-smokers. Hum. Reprod. 2010.