Some medicines or medical conditions may interact with this medicine. INFORM YOUR DOCTOR OR PHARMACIST of all prescription and over-the-counter medicine that you are taking. DO NOT TAKE THIS MEDICINE IF you are taking nitrates (eg, isosorbide, nitroglycerin) in any form (eg, tablet, capsule, spray, patch, ointment), nitroprusside, certain recreational drugs called "poppers" (eg, amyl nitrate or nitrite, butyl nitrate or nitrite), other medicines that contain sildenafil, other PDE5 inhibitors (eg, tadalafil, vardenafil), or ritonavir. ADDITIONAL MONITORING OF YOUR DOSE OR CONDITION may be needed if you are taking alpha-blockers (eg, doxazosin), medicines for high blood pressure, azole antifungals (eg, itraconazole, ketoconazole), H2 antagonists (eg, cimetidine), HIV protease inhibitors (eg, saquinavir), macrolide antibiotics (eg, erythromycin), telithromycin, barbiturates (eg, phenobarbital), bosentan, carbamazepine, efavirenz, nevirapine, phenytoin, rifabutin, or rifampin. DO NOT START OR STOP any medicine without doctor or pharmacist approval. Inform your doctor of any other medical conditions, including certain blood cell problems (eg, leukemia, multiple myeloma, sickle cell anemia), a deformed penis (eg, cavernosal fibrosis, Peyronie disease), or any other condition that may increase the risk of a prolonged (more than 4 hours) or painful erection (priapism); pulmonary hypertension caused by sickle cell anemia; dehydration; allergies; pregnancy; or breast-feeding. Tell your doctor if you have a history of certain eye problems (eg, macular degeneration, optic neuropathy, retinitis pigmentosa, sudden vision loss in one or both eyes), hearing problems (eg, ringing in the ears, decreased hearing, hearing loss), liver or kidney problems, high or low blood pressure, ulcers, bleeding problems, heart problems (eg, aortic stenosis, angina, irregular heartbeat, heart failure), blood vessel problems, priapism, or nervous system problems. Tell your doctor if you have a history of heart attack, stroke, or life-threatening irregular heartbeat, especially within the past 6 months. Tell your doctor if you are taking a medicine for erectile dysfunction (ED). USE OF THIS MEDICINE IS NOT RECOMMENDED if you have pulmonary veno-occlusive disease (PVOD). USE OF THIS MEDICINE in children is not recommended. Discuss with your doctor the risks and benefits of giving this medicine to your child. Contact your doctor or pharmacist if you have any questions or concerns about using this medicine.