Some men will need to start with a lower dose of 25mg prescribed by a doctor — see below. This causes the penis to get hard and erect. These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects. This medicine is available on prescription and you can also buy it from most pharmacies. Severe liver disease.Viagra is used to treat erectile dysfunction (impotence) in men. Another brand of sildenafil is Revatio, which is used to treat pulmonary arterial hypertension and improve . Since , Viagra (sildenafil) has been used for treating erectile dysfunction (ED). It’s available by prescription for males ages 18 years and older. Side effects are . Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes .