Follow your doctor’s orders or the directions on the label. Side Effects Along with its needed effects, a medicine may cause some unwanted effects. Take sildenafil as needed before sexual activity. Tell your doctor if any of these symptoms are severe or do not go away: headache. To use the oral liquid: Shake the bottle well for at least 10 seconds before each use.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. Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial hypertension (PAH; high blood pressure in the vessels carrying blood to the . Dosage and strength for pulmonary hypertension. Sildenafil tablets for pulmonary hypertension come in 10mg and 20mg strengths. The usual adult dose is 20mg taken 3 times a day. Doses for children can range from 10mg to 20mg taken 3 times a day, depending on their weight. Do not take it more than 3 times a day. How to take tablets.