Sexual activity may be a strain on your heart, especially if you have heart disease. What special dietary instructions should I follow? You can take sildenafil with or without food. You should only be treated with one of these products at a time. Alpha-blockers are sometimes prescribed for prostate problems or high blood pressure.Sildenafil (Viagra) is a medicine used to treat erectile dysfunction (ED, also called impotence). Sildenafil works by helping to relax the blood vessels in the penis, allowing more blood to flow in and cause an erection. You should take sildenafil about an hour before planned sexual activity, so it has time to work. See locations near you by entering a ZIP code, or city and state ZIP code, or city and state. Find a clinic. Reschedule or cancel an existing visit Don’t want to leave the house? . CHOOSE BRAND-NAME VIAGRA. The original little blue pill prescribed by doctors for over 20 years. Clinically proven to treat men with erectile dysfunction (ED) Ongoing .