Propecia, popular as a male pattern baldness treatment, has been linked to reports of erectile dysfunction and other sexual side effects.

Recent studies show these Propecia side effects can last long past cessation of treatment, and some raise the question of whether they might be permanent.

Propecia is Merck’s brand name for finasteride, the same drug sold under the name Proscar as a treatment for enlargement of the prostate. Finasteride works by inhibiting the activity of the enzyme necessary for the conversion of testosterone to dihydrotestosterone.

Reducing dihydrotestosterone in the prostate tends to help resolve prostate enlargement. Reducing the same hormone in balding scalp tends to treat male pattern hair loss.

Since its initial FDA approval in 1997, Propecia has been a business success, with annual sales in the hundreds of millions. However, there have been some reports of sexual side effects occurring in association with Propecia treatment.

Probably the most notorious Propecia side effect is erectile dysfunction, but users have reported a variety of other related side effects such as trouble reaching orgasm, loss of libido, and abnormalities in the semen.

Sometimes these Propecia side effects last long after the user has ceased Propecia treatment. Other users have also reported non-sexual side effects including depression and loss of memory.

Clinical studies of Propecia show results consistent with these anecdotal reports. One study published in the Journal of Sexual Medicine in 2011 studied the experiences of 71 men who suffered sexual side effects that persisted after ceasing finasteride treatment. Of these men, 94 percent reported low sex drive, 92 percent reported erectile dysfunction, 92 percent reported decreased arousal, and 69 percent developed problems achieving orgasm. On average, these men’s symptoms persisted for 40 months after ceasing finasteride treatment.

A study conducted through George Washington University found similar results. Out of 54 men who reported persistent sexual side effects three months after finasteride treatment, 89 percent of them continued to have symptoms that met the definition of sexual dysfunction 14 months later. The head researcher of that study speculated that finasteride could be causing permanent damage.

In April 2012, the FDA announced it would require the makers of both Propecia and Proscar to expand the warnings on the labels for those drugs. For Propecia, the FDA required the label to mention reports of “libido disorders, ejaculation disorders, and orgasm disorders that continued after discontinuation of the drug” and also reports of male infertility and poor semen quality that resolved after discontinuing Propecia.

In preparing this label change, the FDA reviewed 421 reports of sexual dysfunction associated with Propecia, all submitted to the FDA’s Adverse Event Reporting System between 1998 and 2011. Fifty-nine of those reported Propecia side effects that persisted for at least three months after discontinuing Propecia.

For Propecia patients concerned about the possibility of sexual side effects, the FDA recommends they consult their physician directly. The agency does not recommend patients discontinue the medication without talking to their doctor first.