This medicine works best when you take it on an empty stomach an hour before sex. This increased blood flow can cause an erection. This can happen if you are sick with fever, diarrhea, or vomiting; sweat a lot; or do not drink enough liquids. Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors. Children 1 year of age and older weighing 20 to 45 kg—20 mg 3 times a day. Sexual activity may be a strain on your heart, especially if you have heart disease. · The cost of sildenafil without insurance starts at $* for a quantity of 30, 20mg tablets, using a RxSaver coupon. The cost of sildenafil can vary in price depending on the pharmacy your sildenafil prescription is filled at. Below is a list of pharmacies and the starting cost of a sildenafil prescription using an RxSaver coupon. – starting at $6 per dose of sildenafil or $16–$22 per month for a day supply: 20– mg: REX MD: yes: yes – $90 per dose of Viagra – $10 per dose of sildenafil: 25– mg.  · For oral dosage forms (suspension or tablets): Adults—20 milligrams (mg) 3 times a day. Your doctor may adjust your dose as needed and tolerated. However, the dose is usually not more than 80 mg 3 times a day. Children 1 year of age and older weighing 45 kilograms (kg) or more—20 mg 3 times a day.