Prolonged use of two treatments for enlarged prostate are associated with persistent erectile dysfunction that can last months or even years after the therapies have been stopped.

The study, “Persistent erectile dysfunction in men exposed to the 5α-reductase inhibitors, finasteride, or dutasteride,” was published in the scientific journal PeerJ.

The therapies, Proscar or Propecia (finasteride) and Avodart (dutasteride), inhibit an enzyme called 5α-reductase. The treatments prevent the sex hormone testosterone from being converted into the more powerful 5α-dihydrotestosterone (5α-DHT).

Finasteride is prescribed not just for benign prostatic hyperplasia, but also for androgenic alopecia, or male-pattern baldness. Although the U.S. Food and Drug Administration (FDA) has not approved dutasteride for baldness, many men use it for that.

The manufacturers’ prescription information says longtime use of the drugs does not increase the risk of sexual adverse side effects. It also says adverse side effects disappear once the therapies are discontinued. This does not appear true, however, researchers said.

“Our study shows men who take finasteride or dutasteride can get persistent erectile dysfunction, in which they will not be able to have normal erections for months or years after stopping finasteride or dutasteride,” the senior author of the study, Dr. Steven Belknap, said in a news release.

The team analyzed 11,909 men, ages 16 to 89, who took either finasteride or dutasteride. They wanted to know whether the men had persistent erectile dysfunction — that is, dysfunction lasting 90 days or more after the medication ended.

Researchers used men who had shorter periods of treatment as comparison controls with those who used them longer.

Five hundred thirty of the 11,909 men, or 4.45%, had new cases of erectile dysfunction. One hundred sixty-seven of the 530 had erectile dysfunction that continued an average of 1,348 days — or more than 3.5 years — after they stopped the treatment.

Men without an enlarged prostate were at almost five times higher risk of persistent erectile dysfunction when they used non-steroidal anti-inflammatory drugs and finasteride or dutasteride for more than 208.5 days.

Doctors prescribed low doses of finasteride — 1.25 mg or less per day — to 4,284 men between 16 and 42. Thirty-four, or 0.8%, developed persistent erectile dysfunction lasting on average of 1,534 days — more than four years.

Compared with men with shorter finasteride exposure, young men who used finasteride more than 205 day were at almost five times higher risk of persistent erectile dysfunction.

“The new findings of an association between debilitating sexual dysfunction and exposure to finasteride or dutasteride should be of particular interest to prescribers and patients considering medical management of androgenic alopecia or symptomatic treatment of enlarged prostate,” Belknap concluded.