If you have these symptoms, stop taking VIAGRA and contact a doctor right away The most common side effects of VIAGRA are headache; flushing; upset stomach; abnormal .Severe liver disease. Continue to take sildenafil even if you feel well. Sildenafil does not prevent pregnancy or the spread of sexually transmitted diseases such as human immunodeficiency virus HIV. Do not stop taking sildenafil without talking to your doctor. They include dutasteride Avodart, Jalyn and finasteride Propecia, Proscar. Sildenafil Viagra.If you have these symptoms, stop taking VIAGRA and contact a doctor right away. Before you take VIAGRA, tell your healthcare provider if you: • have or have had heart problems . Oral medicines for ED usually work well and cause few side effects. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil . Learn about uses, how it works, and more for Viagra (sildenafil). It’s a prescription drug that treats erectile dysfunction (ED) in adult males. Health Conditions.If you have an erection that lasts more than 4 hours, get medical help right away. These medicines are called PDE5 inhibitors. Brand names Why is this medication prescribed? The following nonprescription or herbal products may interact with sildenafil: cimetidine Tagamet HB , St. They include dutasteride Avodart, Jalyn and finasteride Propecia, Proscar. It lasts 4 to 5 hours, or longer if you have mild to moderate ED.