Two drugs—one to treat hair loss, the other to shrink enlarged prostates—could wreak havoc on men's sexual health. Researchers at Northwestern University reported Thursday in the journal PeerJ that among the men they studied who are between the ages of 16 and 89 and who took either finasteride (for their hair or prostate) or dutasteride (for their prostate), 1.4% developed persistent erectile dysfunction (PED) that lasted a median of 3.7 years—after they stopped taking the drug. Both drugs block testosterone from converting into 5-alpha-dihydrotestosterone, or 5a-DHT, which mutes a chemical signal that fuels hair-cell growth. But it could be muting other important signals involved in achieving and maintaining erections.
For those under age 42, how long they took drug seemed to influence how long the effect lasted: Those who used one of the drugs for more than 205 days had 4.9 times higher risk of PED than those under the 205-day window. In fact, the researchers found "exposure was a more accurate predictor of PED" than many known risk factors, including age, diabetes, and obesity. Exposure was also a more accurate predictor than dosage, "likely reflecting that finasteride exerts near-maximal inhibition of 5a-DHT synthesis at a dose of 1 mg," which is the dosage indicated for hair-loss treatment. The same researchers concluded in a 2015 study in JAMA Dermotology that there's insufficient evidence that the drug finasteride is safe to take to treat hair loss in men, reports LiveScience. (Read about the very odd origins of finasteride.)