* A decrease in sexual desire or ability * Difficulty achieving an erection or impotence * Pain or discomfort in the testicles * Breast tenderness or enlargement It is important to talk to your doctor if you experience any of these side effects. In rare cases, Propecia can also cause more serious side effects such as allergic reactions or liver damage. If you have any concerns about taking Propecia, it is recommended that you speak with a healthcare professional before starting the medication.