Sildenafil is also sometimes used to treat pulmonary hypertension high blood pressure in the blood vessels that supply the lungs. Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors. Read and follow these instructions carefully before you start using sildenafil and each time you get a refill of your medicine. It comes as standard tablets, chewable tablets, and as a liquid that you swallow. Sildenafil should not be used with any other medicine or device that causes erections. The following nonprescription or herbal products may interact with sildenafil: cimetidine Tagamet HB , St.Generic Viagra (sildenafil) is available as a , , , and milligram (mg) pill and in liquid form. It’s meant to be taken as needed — about an hour before sex — but shouldn’t be. Virtual Care Don’t want to leave the house? Connect with a licensed provider 7 days a week from the comfort of home. Explore Virtual Care Loading MinuteClinic can treat over . This medicine is available on prescription and you can also buy it from most pharmacies. It comes as standard tablets, chewable tablets, and as a liquid that you swallow. Key facts. It usually takes 30 to 60 minutes for sildenafil to work for erectile dysfunction. You can take it up to 4 hours before you want to have sex.