Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. You can take this medicine with or without food 30 minutes before sex, depending on the dose. Vardenafil usually works for 4 to 5 hours. Visiting a specialist Sometimes ED is a symptom of an underlying condition. The dose of Viagra would need to be adjusted with this in mind.Why is this medication prescribed? Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Viagra relaxes muscles found in the walls of blood vessels and increases blood flow to particular areas of the body. Viagra is used to treat erectile dysfunction (impotence) in . Why you need a prescription for Viagra Does Viagra require a prescription? Yes, and it’s because you’ll need to work with a healthcare provider to monitor for side effects and .