You can take it up to 4 hours before you want to have sex. Remove the syringe from the bottle. However, the dose is usually not more than 80 mg 3 times a day. How and when to take sildenafil It’s important to take sildenafil as advised by a doctor or pharmacist. Dosage and strength for pulmonary hypertension Sildenafil tablets for pulmonary hypertension come in 10mg and 20mg strengths.You can take the mg and mg doses as shortly as 15 minutes before sexual activity. As a second-generation medication, Stendra also seems to have milder side effects than Viagra and the. Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). The drug comes as a tablet you’ll take by swallowing. It’s usually taken as needed about 1 hour. Examples of Viagra’s more commonly reported side effects include: headache. flushing (temporary warmth, redness, or deepening of skin color) stuffy nose. indigestion (upset stomach) mild vision.