Propecia (low-dose finasteride, 1 mg) is a hair-loss treatment for many men (estimated 4 million) of reproductive age, and higher dose (5 mg) finasteride is already known to reduce multiple semen parameters (count, movement, semen volume). These semen changes could be related to an effect on intratesticular hormonal milieu, sperm DNA integrity, or epididymal function.
For men taking Propecia, some interesting information about possible effects on fertility have been noted recently. Among men with normal fertility, no effect of low-dose finasteride has been noted. The question answered here is what happens to men who may have an infertility issue (and not realize it) and take Propecia.
A study from the University of Toronto of 24 subfertile men noted that men taking 1 mg of finasteride for 5 years noted an 11 fold increase in sperm counts 3 months after finasteride discontinuation. Of the men with sperm count <5 million/ml, more than half of the men increased counts above 15 million/ml.
More studies are needed to confirm this possible reversible cause of low sperm count and why it effects some men more than others. In the meanwhile, if having fertility problems and taking Propecia, consult with your physician.