There's no definitive proof that either Propecia or Proscar actually causes these problems. But the FDA has received enough reports of these side effects to cause concern.
Even if the drugs do cause these sexual side effects, they've been reported in only a small number of men. For example, in Propecia clinical trials, 3.8% of men taking the drug and 2.1% of men getting an inactive placebo reported sexual problems.
Moreover, the warnings are based on relatively small numbers of reported problems.
For Propecia, the FDA reviewed 421 reports of sexual dysfunction between 1998 and 2011. Of these, 59 cases reported sexual dysfunction that lasted for at least three months after patients stopped using the drug.
The FDA identified 251 cases of poor semen quality linked to Propecia and 29 cases linked to Proscar use. Interestingly, only three of the Proscar-linked cases were in men with BPH.
Patients taking either Propecia or Proscar should talk with their doctors if they are worried about the new warnings. Men suffering sexual dysfunction or having fertility problems should consult their doctors and report the side effects to the FDA MedWatch program either online or at 800-332-1088.
Proscar and Propecia are both brand-name drugs from Merck. Generic versions of both drugs are available.