Regular Ibuprofen Use May Contribute to Male Infertility, Study Says

Regularly taking common over-the-counter painkillers may contribute to male infertility, a problem that affects roughly 7% of American men, according to new research.

A study published Monday in Proceedings of the National Academy of Sciences found that regular ibuprofen use may lead to compensated hypogonadism, a condition that can lead to infertility, erectile dysfunction, depression and loss of bone and muscle mass, among other symptoms. The condition is most commonly seen in smokers and the elderly, but the new research suggests it can affect young men, too.

The researchers recruited 31 healthy men between the ages of 18 and 35. Fourteen of the men took two 600-milligram doses of ibuprofen per day for six weeks — an amount consistent with what many athletes take to manage aches and pains — while the remaining 17 took placebo pills. (Ibuprofen is the generic name for drugs including Advil and Motrin. Tylenol, meanwhile, is a brand name for a different drug, acetaminophen.)

Both groups of men submitted to blood tests and hormonal analysis throughout the study. After 14 days of ibuprofen use, the researchers observed higher blood levels of luteinizing hormone, which regulates the production of testosterone and other hormones. After 44 days, levels were even higher. Testosterone production, however, did not increase concurrently, resulting in a lower ratio of testosterone to luteinizing hormone — a sign of hypogonadism, according to the paper.

The researchers also observed other hormonal disruptions at 14 and 44 days of ibuprofen consumption, suggesting wide-ranging consequences of hypogonadism.

Next, the researchers tested the direct effect of ibuprofen on testicles, using samples that had been taken from organ donors. When exposed to levels of ibuprofen similar to that which would be taken orally, the testicle samples produced less testosterone after just 24 hours. The higher and longer the level of exposure, the researchers found, the more dramatic the impact. Gene expression associated with turning cholesterol into steroidal hormones was also impaired, they found.

Prior research from the study’s lead author also found that boys born to mothers who took ibuprofen during their first trimester of pregnancy may have impaired testicular development, adding weight to the idea that, at least in some scenarios, the drug may have a negative effect on fertility.

The Consumer Healthcare Products Association, which represents the manufacturers of over-the-counter medicines, takes issue with the study’s small size and the long duration of ibuprofen use. “Every day, millions of Americans rely on over-the-counter (OTC) ibuprofen to treat pain and/or fever,” the group says in a statement provided to TIME, “and they can rest assured that the safety and efficacy has been well documented and supported by decades of scientific study and real-world use.”