It lasts 4 to 5 hours, or longer if you have mild to moderate ED. To use the oral liquid: Shake the bottle well for at least 10 seconds before each use. Sorry something went wrong with your subscription Please, try again in a couple of minutes Retry. Do not take other medicines unless they have been discussed with your doctor. Children 1 year of age and older weighing 20 to 45 kg—20 mg 3 times a day. · Adults 65 years of age and older—25 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. Your doctor may adjust your dose if needed. Children—Use is not recommended. For treatment of pulmonary arterial hypertension. The typical starting dosage of Viagra for a year-old would be 25 milligrams (mg). For a year-old who has no other medical conditions, the typical starting dosage would be 50 mg. Strengths: 25 mg, 50 mg, mg; Brand: Viagra. Form: oral tablet; Strengths: 25 mg, 50 mg, mg; Adult dosage (ages 18–64 years) Typical dosage: 50 mg, taken as needed about 1 hour before.