Ask your doctor if you have any questions. Take sildenafil up to 4 hours before you want to have sex. It lasts 4 to 5 hours, or longer if you have mild to moderate ED. Continue to take sildenafil even if you feel well. May make this condition worse. If you are taking sildenafil to treat PAH, you will probably take sildenafil three times a day with or without food. If you become pregnant while taking sildenafil, call your doctor.Dosage for erectile dysfunction The recommended dosage of Viagra for erectile dysfunction (ED) is 50 mg, taken as needed about 1 hour before sexual activity. You can take it 30 . Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in . Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or alcohol. Its effects .