If you want to buy this medicine online, make sure that you choose a reputed company and one that has a good reputation. If you need emergency medical care for a heart problem, it is important that your doctor knows when you last took sildenafil. However, the dose is usually not more than 80 mg 3 times a day. Oral medicine for ED also might not be safe or work well if you have: Very low blood pressure, called hypotension, or uncontrolled high blood pressure, called hypertension. Doses for children can range from 10mg to 20mg taken 3 times a day, depending on their weight. It is not known if the medicine is safe for use in these patients. · Take sildenafil exactly as it was prescribed for you. Follow all directions on your prescription label and read all medication guides or instruction sheets. Revatio is usually taken three times each day, about 4 to 6 hours apart. Viagra is usually taken only when needed, 30 minutes to 1 hour before sexual activity. What would happen if I took a larger dose of Viagra than my doctor prescribed — for example, mg or mg? You shouldn’t take more Viagra than your doctor prescribes. The maximum.  · Use only the brand of this medicine that your doctor prescribed. Different brands may not work the same way. Dosing. 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 20 to 45 kg—20 mg 3 times a day.