Adults 65 years of age and older—25 mg as a single dose no more than once a day, 1 hour before sexual intercourse. Place the tip of the syringe into you or your child’s mouth and point it towards the cheek. Keep out of the reach of children. Sildenafil works on the PDE5 enzyme in the lungs to relax the blood vessels. You’ll soon start receiving the latest Mayo Clinic health information you requested in your inbox. If you have an erection that lasts more than 4 hours, get medical help right away. From Mayo Clinic to your inbox Sign up for free and stay up to date on research advancements, health tips, current health topics, and expertise on managing health.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 .  · 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 .  · Sildenafil is used to treat men who have erectile dysfunction (also called sexual impotence). Sildenafil belongs to a group of medicines called phosphodiesterase .