Dear Dr. Roach: I am a 70-year-old man and have been taking finasteride for hair loss for about five years. I realized while reading your recent column that I have the sexual side effects the letter-writer described. I had been unable to obtain a satisfactory erection even when taking 150 mg of sildenafil, which is three times my prescribed dose. Ejaculation was extremely rare and difficult, with low semen volume and a feeling of dissatisfaction, to say the least. I immediately stopped taking finasteride the day after reading your column, about two months ago. I have recovered some of the ability to have an erection with 100 mg of finasteride, but I have not recovered the semen volume or a feeling of satisfaction with ejaculation.

How long do the side effects typically last? I have seen answers to this question varying from three to 40 months. Also, is there a possibility that side effects are underreported? I don’t even know how to report side effects. I do not remember ever having to stop medication due to side effects in my life until now.

— R.H.

Answer: Finasteride and dutasteride work by blocking an enzyme called 5-alpha reductase, which transform testosterone into dihydrotestosterone. Dihydrotestosterone is responsible for most of the prostate enlargement that happens in middle-aged and older men, as well as much of the hair loss that happens in genetically predisposed men. For this reason, these drugs are used both for men with symptomatic enlarged prostate and for men who are starting to go bald. The drug also reduces your risk of prostate cancer but is rarely prescribed for this purpose alone.

Most men do not have side effects with this treatment. However, an average of 1.5% of men over 9 studies had sexual side effects, including loss of libido, erectile dysfunction and ejaculatory dysfunction. It is possible this is underreported (some individual studies reported numbers of 7% to 8%), but I don’t think it is very much underreported because of the large number of trials looking specifically for this issue.

Finasteride lasts a much shorter time in the body than dutasteride (half of finasteride is gone in 8 hours versus 4 weeks for dutasteride), so any side effects would be expected to resolve faster, too. However, although most people who stopped the medicine due to sexual side effects improved over time, a subset of men had persistent effects that did not improve over time and were not improved with treatment.

Another reader asked about dementia and depression with this class of drugs. A well-done study suggested that dementia risk is probably not significant; however, depression has also been reported with treatment. Depression resolved between three days and three months after stopping the medication.

While it is possible that people who expect side effects may get them (a phenomenon known as the “nocebo effect”), it seems clear from the literature that these drugs may cause both sexual and mood side effects, and it is wise to discuss these possibilities prior to prescribing.

Contact Dr. Roach at ToYourGoodHealth@med.cornell.edu.