Recent heart or blood vessel problems. Do not take more or less of it or take it more often than prescribed by your doctor. If you prefer, you may contact the U. Rarely reported side effects include: an erection that will not go away priapism. If you are taking sildenafil for erectile dysfunction, your doctor will probably start you on an average dose of sildenafil and increase or decrease your dose depending on your response to the medication. If you ever need emergency medical treatment for a heart problem, the healthcare providers who treat you will need to know when you last took sildenafil. You are encouraged to report adverse events related to Viatris products by calling U.Your doctor may first recommend proven ED medications, such as tadalafil (Cialis) and sildenafil (Viagra). Tadalafil can be effective up to 36 hours after taking it. Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles . Overdose Ask your doctor Q&A Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription .