These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects. Often, these are prescribed for chest pain called angina. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide. Viagra vs.tadalafil (Cialis) vardenafil (Levitra) avanafil (Stendra) About 30 million men in the United States occasionally have a problem with getting or keeping an erection, . Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription drug that’s used in adult . Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body .