If you have certain health conditions or are taking certain medications, your doctor may tell you to take sildenafil less often. When choosing one, tell your doctor or other health care professional what your preferences are, such as cost, ease of use, how long the drug’s effects last and side effects. These include a heart attack, stroke or a serious heart rhythm disorder in the past six months. Sildenafil may cause side effects. Common ones include: Flushing of the skin.Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide. This boosts blood flow and helps you get an erection from sexual activity. How oral medicines differ. Viagra comes as an oral tablet that contains the active drug sildenafil. Sildenafil is also available in generic form. Viagra is classified as a phosphodiesterase type 5 (PDE5) inhibitor. Sildenafil is used to treat male sexual function problems (impotence or erectile dysfunction-ED). In combination with sexual stimulation, sildenafil works by increasing blood flow to the penis to.