You can take it in a small dose daily or in a larger dose as needed. These include a heart attack, stroke or a serious heart rhythm disorder in the past six months. Medicines that block an enzyme called CYP3A4. Alpha-blockers are sometimes prescribed for prostate problems or high blood pressure. The information provided is for educational purposes only and is not intended to replace discussions with a healthcare provider. It works for up to 36 hours.Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes . CHOOSE BRAND-NAME VIAGRA. The original little blue pill prescribed by doctors for over 20 years. Clinically proven to treat men with erectile dysfunction (ED) Ongoing . 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 .