Alternatively, the medicine may be taken 30 minutes to 4 hours before sexual intercourse. Storage Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light. It continues to work for up to 4 hours, although its action is usually less after 2 hours. Store mixed oral liquid in the refrigerator. These medicines prevent an enzyme called phosphodiesterase type-5 from working too quickly. You’ll soon start receiving the latest Mayo Clinic health information you requested in your inbox.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 .  · 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. . VIAGRA is available by prescription only in 25 mg, 50 mg, and mg tablets. Patients should always ask their doctors for medical advice about adverse events. You are .