This medicine also works best when you take it an hour before sex on an empty stomach. Medicines that you take by mouth are called oral medicines. Rarely reported side effects include: an erection that will not go away priapism. Generic versions of sildenafil, vardenafil and tadalafil are available. Its effects often last for 4 to 5 hours.Viagra generic form. Viagra is a brand-name medication. Viagra vs. over-the-counter products. Many over-the-counter (OTC) products may be advertised . By Ginger Skinner Last updated: June 14, If you’re looking for a less costly Viagra, you might want to ask your doctor about sildenafil. It’s the active ingredient . Cart. Transfer your prescription for Sildenafil (Generic for Viagra, Oral Tablet) on Amazon Pharmacy. FREE SHIPPING for Prime members.