Tell your doctor if sildenafil is not working well or if you are experiencing side effects. It continues to work for up to 4 hours, although its action is usually less after 2 hours. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine. Sildenafil Viagra is used to treat erectile dysfunction impotence; inability to get or keep an erection in men. You’ll soon start receiving the latest Mayo Clinic health information you requested in your inbox. Children 1 year of age and older weighing 45 kilograms kg or more—20 mg 3 times a day.The normal dose when starting Viagra is 50 mg, as needed. The maximum dose of Viagra is mg each day. Doses above the maximum dose, such as mg .  · This product is available in the following dosage forms: Tablet; Powder for Suspension; Suspension. Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in .