Take sildenafil as needed before sexual activity. Do not take a double dose to make up for a missed one. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. These medicines all work well for ED. Oral medicine for ED also might not be safe or work well if you have: Very low blood pressure, called hypotension, or uncontrolled high blood pressure, called hypertension.Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial . The active drug in Viagra is sildenafil. It belongs to a group of drugs called phosphodiesterase-5 inhibitors. If you’ve had an allergic reaction to Viagra or any of . Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes .