Decreased sex drive, trouble getting or keeping an erection, or decreased amount of semen during sex may occur. Headache, upset stomach, back pain, muscle pain, stuffy nose, flushing, or dizziness may also occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly.To reduce the risk of dizziness and lightheadedness, get up slowly when rising from a sitting or lying position.Remember that this medication has been prescribed because your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.Tell your doctor right away if you have any serious side effects, including: breast changes (such as enlargement, lumps, pain, nipple discharge), pain in the testicles, color vision changes.Sexual activity may put extra strain on your heart, especially if you have heart problems. If you have heart problems and experience any of these serious side effects while having sex, stop and get medical help right away: chest/jaw/left arm pain, shortness of breath, dizziness, nausea.Rarely, sudden decreased vision, or even permanent blindness, may occur in one or both eyes (NAION). If this serious problem occurs, stop taking this medication and get medical help right away. You have a slightly greater chance of developing NAION if you have heart disease, diabetes, high cholesterol, certain other eye problems ("crowded disk"), high blood pressure, if you are over 50, or if you smoke.Rarely, a sudden decrease or loss of hearing, sometimes with ringing in the ears and dizziness, may occur. Stop taking this medication and get medical help right away if these effects occur.Rarely, males may have a painful or prolonged erection lasting 4 or more hours. If this occurs, stop using this drug and get medical help right away, or permanent problems could occur.A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing.This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.In the US -Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.