Children 1 year of age and older weighing 45 kilograms kg or more—20 mg 3 times a day. That means seven out of 10 people who take these drugs as prescribed will have more frequent or longer-lasting erections. If you see air bubbles in the syringe, repeat the steps. Usually, it takes between 30 and an hour for Viagra to start working, and lasts for four to six hours. But the drug has proven effective for many men, making it one of the most well-liked and well-known therapies for erectile dysfunction. Tadalafil Cialis. · For treatment of erectile dysfunction: For oral dosage form (tablets): Adults up to 65 years of age—50 milligrams (mg) as a single dose no more than once a day, 1 hour before sexual intercourse. Alternatively, the medicine may be taken 30 minutes to 4 hours before sexual intercourse.  · Generic name: sildenafil (oral) [ sil-DEN-a-fil ] Brand names: Revatio, Viagra Drug classes: Agents for pulmonary hypertension, Impotence agents Medically reviewed by Kaci Durbin, MD. Last updated on Uses Warnings Before taking Dosage Side effects Interactions FAQ What is sildenafil? For men experiencing erectile dysfunction (ED), exercising regularly can be nearly as effective as ED medications, such as Viagra (sildenafil), in improving performance, according to a new meta-study.