Recent heart or blood vessel problems. This medication is taken with or without food an hour before sex. Severe liver disease. The penis is one of the areas where this enzyme works. You’ll soon start receiving the latest Mayo Clinic health information you requested in your inbox.Viagra takes around 30 minutes to become effective and lasts about 4 hours. Cialis lasts much longer — up to 36 hours in some cases. Stendra can start doing its thing in as little . · Viagra; Descriptions. Sildenafil is used to treat men who have erectile dysfunction (also called sexual impotence). Sildenafil belongs to a group of medicines . Last updated: June 14, If you’re looking for a less costly Viagra, you might want to ask your doctor about sildenafil. It’s the active ingredient in both branded Viagra and the drug.