Medicines that you take by mouth are called oral medicines. These medicines are called PDE5 inhibitors. Medicines that block an enzyme called CYP3A4. Its effects often last for 4 to 5 hours. Generic versions of sildenafil, vardenafil and tadalafil are available. Vardenafil usually works for 4 to 5 hours.Generic Viagra (sildenafil) is an alternative to the branded Viagra drug that can be used for treating erectile dysfunction (ED). It’s now sold by many manufacturers, including online. How we vet brands and products. If you’re interested in buying Viagra online, five of the best options available are Hims, Roman, Lemonaid, Optum Store, and REX MD. Read on for . Valid at all major chains including Walgreens, CVS Pharmacy, Target, WalMart Pharmacy, Duane Reade and 65, pharmacies nationwide. No manufacturer promotions could be .