They’re often the first line of treatment for trouble getting or keeping an erection, called erectile dysfunction ED. Other uses for this medicine This medication is sometimes prescribed for other uses; ask your doctor or pharmacist for more information. What should I do if I forget a dose? Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors. Avanafil Stendra.Sildenafil (Viagra brand) increases blood flow to the penis following sexual stimulation. It does this by blocking the enzyme responsible for the breakdown of cGMP. . 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 . Learn about uses, how it works, and more for Viagra (sildenafil). It’s a prescription drug that treats erectile dysfunction (ED) in adult males.