It works for up to 36 hours. Often, these are prescribed for chest pain called angina. Medicines that block an enzyme called CYP3A4. Other uses for this medicine This medication is sometimes prescribed for other uses; ask your doctor or pharmacist for more information. Brand names Why is this medication prescribed? This can happen if you are sick with fever, diarrhea, or vomiting; sweat a lot; or do not drink enough liquids. This medicine works best when you take it on an empty stomach an hour before sex.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 . How does Viagra work? Viagra is used to treat erectile dysfunction (ED). It decreases ED symptoms by helping you get and keep an erection. Erections happen . Take sildenafil as needed before sexual activity. 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 .