If you prefer, you may contact the U. You can take sildenafil with or without food. The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity. Before starting, stopping, or changing any medications while taking sildenafil, please get the advice of your doctor or pharmacist. Recent heart or blood vessel problems. You are encouraged to report adverse events related to Viatris products by calling U. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide.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 adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial hypertension (PAH; high blood pressure in the vessels carrying blood to the . Viagra is usually taken only when needed, 30 minutes to 1 hour before sexual activity. You may take it up to 4 hours before sexual activity. Do not take Viagra more than once per day. Viagra can help you have an erection when sexual stimulation occurs. An erection will not occur just by taking a pill. Follow your doctor’s instructions. Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide. This boosts blood flow and helps you get an erection from sexual activity. How oral medicines differ.