How can finasteride cause testicular pain?

Unknown.: Testicular pain is an uncommon side effect reported in patients taking finasteride. In phase 3 clinical trials, less than 1% had this side effect. The mechanism of this pain is unknown. Finasteride blocks the formation of dht, an active form of testosterone by inhibiting 5-alpha reductase type ii. Since the testicles are under hormonal control, the mechanism of pain may involve this pathway.