This can happen if you are sick with fever, diarrhea, or vomiting; sweat a lot; or do not drink enough liquids. These medicines are called PDE5 inhibitors. Your doctor or other health care professional can recommend one for you based on these factors, other health conditions you have and any other medicines you take. Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors. Its effects often last for 4 to 5 hours. This medicine also works best when you take it an hour before sex on an empty stomach.Learn about uses, how it works, and more for Viagra (sildenafil). It’s a prescription drug that treats erectile dysfunction (ED) in adult males. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called . Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). Viagra can cause side effects that range from mild to serious. .