It lasts 4 to 5 hours, or longer if you have mild to moderate ED. Sildenafil Viagra. These include a heart attack, stroke or a serious heart rhythm disorder in the past six months. Its effects often last for 4 to 5 hours. Often, these are prescribed for chest pain called angina.Cost 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 . In the U.S. you cannot get Viagra or its generic forms without a doctor’s prescription or over the counter. There are a few good reasons for keeping these medications behind . Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes .