This medication is taken with or without food an hour before sex. It works for up to 36 hours. It lasts 4 to 5 hours, or longer if you have mild to moderate ED. You can take sildenafil with or without food. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily.Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or alcohol. Its effects . 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 . Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). The drug comes as a tablet you’ll take by swallowing. It’s usually taken as needed about 1 .