Oral medicines for ED usually work well and cause few side effects. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. Find patient medical information for Sildenafil (Viagra, Revatio) on WebMD including its uses, side effects and safety, interactions, pictures, warnings, and user ratings You need a prescription to get Viagra in the United States, Canada, and most other countries. Viagra is also available in the generic form sildenafil, which requires a prescription as well. Compare prices, and save at 60,000 pharmacies and Amazon Pharmacy in brand and generic drugs Sildenafil tablets (25 mg, 50 mg, 100 mg; Viagra, generic) and sildenafil oral film (25 mg, 50 mg, 75 mg, 100 mg; Vybrique) are used for ED. Sildenafil tablets (20 mg; Revatio, generic), sildenafil oral suspension (Revatio, generic), and sildenafil injection (Revatio, generic) are used for PAH. Using this medicine with any of the following medicines may cause an increased risk of certain side effects, but using both drugs may be the best treatment for you. In the U.S., you can legally buy generic Viagra (sildenafil) online, but only with a prescription from a licensed healthcare professional. Sildenafil for erectile dysfunction is not an over‑the‑counter (OTC) medicine in the U.S., so you cannot legally buy it without a prescription. There is no generic version, so it’s a little more expensive. A typical dose of avanafil is about 50–100 mg, and it takes about 45 minutes to start working. Avanafil also carries similar risks of. Get detailed information about Viagra including uses, side effects, alternatives, and more. Take sildenafil (Viagra) no more than once a day. Otherwise, it can put you at risk of serious side effects, such as vision loss or low blood pressure. Other more common side effects include headache and flushing. Sildenafil (Viagra) is a brand-name and generic medication.