Medicines that block an enzyme called CYP3A4. These are commonly prescribed for an enlarged prostate or male pattern baldness. It works for up to 36 hours. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. Rarely reported side effects include: an erection that will not go away priapism.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. Your prescription may have been filled with a generic version instead of brand-name VIAGRA. Follow the steps below to help ensure you receive the brand name your doctor prescribed and, if eligible, save on your prescription. Ask Your Doctor. Sildenafil oral tablet is available as the brand-name drugs Viagra and Revatio. It’s also available as a generic drug. Generic drugs usually cost less than the brand-name version. In some.