It works for up to 36 hours. For treatment of pulmonary arterial hypertension: For oral dosage forms suspension or tablets : Adults—20 milligrams mg 3 times a day. Do not press on the plunger of the syringe. If you are taking sildenafil for erectile dysfunction, you are unlikely to miss a dose since this medication is taken as needed, not on a regular dosing schedule. What should I do if I forget a dose?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 . The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called .  · Your doctor may adjust your dose as needed and tolerated. However the dose is usually not more than 40 mg 3 times a day. Children 1 year of age and older weighing .