In recent decades, prompted by concerns that men’s sperm quality is declining, researchers have looked at things they suspect of potentially disrupting the body’s endocrine system – from chemicals in water bottles to WiFi laptops to wearing tight underwear instead of boxers.

You can add ibuprofen to the list.

In a study published Monday in the Proceedings of the National Academy of Sciences, researchers found that a concentrated dose of the over-the-counter painkiller taken by young, healthy men appears to be linked to a testicular condition that typically only appears at middle age and has been linked to infertility.

The experiment involved 31 men under 35 in Denmark and France who were split into two groups, with the first taking 1,200 milligrams of ibuprofen each day for six weeks and the other a placebo. While 1,200 milligrams of ibuprofen is considered on the high side for general aches, pains and fever, it’s not unusual for doctors to prescribe twice that much for athletes.

In two weeks, the concentration of testosterone hormone dropped as ibuprofen concentrations grew in the blood of those taking ibuprofen – resulting in the men having a condition known as hypogonadism. Bernard Jégou, of the French National Institutes of Health and Medical Research, and his co-authors wrote that ibuprofen appears to “affect the hormonal balance in adult men” and “alters human testicular physiology” – creating a situation where the body has to work harder to maintain some degree of normal testosterone.

Studies about environmental factors that may impact male fertility have become more urgent with growing evidence that sperm quality is falling globally. In July 2017, in a large analysis published by the American Society of Andrology confirmed that there appears to have been a 52.4 percent drop in sperm concentration between 1973 and 2011.