Recent heart or blood vessel problems. 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. It works for up to 36 hours. Generic versions of sildenafil, vardenafil and tadalafil are available. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Sildenafil Viagra.What is sildenafil? Sildenafil relaxes muscles of the blood vessels and increases blood flow to particular areas of the body. Sildenafil under the name Viagra is used to treat erectile . Why is this medication prescribed? Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Viagra: This drug is used to treat ED. With ED, you’re unable to get or maintain an erection. Revatio: This drug is used to treat PAH. With PAH, the blood pressure in your lungs is .