These medicines all work well for ED. The following nonprescription or herbal products may interact with sildenafil: cimetidine Tagamet HB , St. This boosts blood flow and helps you get an erection from sexual activity. The four main medicines taken by mouth for erectile dysfunction are: Avanafil Stendra. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. This medicine works best when you take it on an empty stomach an hour before sex. Sildenafil comes as a tablet and suspension liquid to take by mouth.By Ginger Skinner 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 . You should not use Viagra if you are allergic to sildenafil, or: if you take other medicines to treat pulmonary arterial hypertension, such as riociguat (Adempas). if you take nitrates. . Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles .