If you have certain health conditions or are taking certain medications, your doctor may tell you to take sildenafil less often. When choosing one, tell your doctor or other health care professional what your preferences are, such as cost, ease of use, how long the drug’s effects last and side effects. You can take this medicine with or without food 30 minutes before sex, depending on the dose. Sildenafil does not prevent pregnancy or the spread of sexually transmitted diseases such as human immunodeficiency virus HIV. Tell your doctor if any of these symptoms are severe or do not go away: headache.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 . Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). The drug comes as a tablet you’ll take by swallowing. It’s usually taken as needed about 1 . Viagra is classified as a phosphodiesterase type 5 (PDE5) inhibitor. For information on the dosage of Viagra, including its pill form, strengths, and how to take the drug, keep .