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. This condition may require prompt medical treatment to prevent serious and permanent damage to your penis. Side effects Side effects of oral ED medicines are often mild. Sudden vision loss in one or both eyes can be a sign of a serious eye problem called non-arteritic anterior ischemic optic neuropathy NAION. Vardenafil usually works for 4 to 5 hours. · Viagra dosage Viagra form. Viagra comes as tablets that you swallow. Viagra strengths. Typical dosages. The following information describes commonly recommended dosages for ED. However, be sure to take the Long-term use. Viagra is meant to be used as a long-term treatment. If you and your. Q&A. Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). Viagra can cause side effects that range from mild to serious. Examples include low blood pressure. Sildenafil (Viagra) is a phosphodiesterase-5 (PDE-5) inhibitor. PDE-5 is a protein in your body that breaks down the molecules that normally cause an erection. When it blocks PDE-5, sildenafil (Viagra) helps an erection occur by allowing certain muscles in the penis to relax and the penis to fill with blood.