The safety of finasteride has been well demonstrated in clinical studies. Nonetheless, <2% of men experienced some form of sexual dysfunction in the clinical trials – A small number of men experience lower sexual desire and/or difficulty in achieving/maintaining an erection. An even smaller number may have a decrease in semen volume. The sexual side effects generally resolved in men who stopped taking finasteride and in many men who continued treatment.

Source:
Propecia product information. U.S. Food & Drug Administration.
Health Science Authority, Singapore