Decreased Fertility in Male Rats Administered the 5α-Reductase Inhibitor, Finasteride, Due to Deficits in Copulatory Plug Formation

Administering finasteride, a 5α-reductase inhibitor, to sexually mature male rats caused a 30-40% decrease in fertility due to deficits in copulatory plug formation. The rats did not form or formed small and improperly positioned copulatory plugs, which are required in rats to transport sperm into the uterus. The decreased fertility was related to decreased weight of seminal vesicles and prostate, an expected pharmacologic effect. The study suggests that decreased fertility in male rats after finasteride administration is a species-specific effect and not likely to be relevant to species that do not form copulatory plugs.