Sildenafil comes as a tablet and suspension liquid to take by mouth. Ask your pharmacist for a list of the ingredients. If you select the Active plan, you will receive six free 30 mg Sildenafil chewable tablets for the first month. This medicine usually begins to work for erectile dysfunction within 30 minutes after taking it. Adults 65 years of age and older—25 mg as a single dose no more than once a day, 1 hour before sexual intercourse. · Updated 04/04/ Yesterday was a big day for you — you got your first prescription for Viagra®. You got it home, took your pill, called your partner to take it on a test drive, and you’re not really impressed. In fact, it feels like this stuff might not have actually worked. What gives? mg of Sildenafil (Viagra) is the highest dose prescribed. Your doctor decides what treatment is best for you. Different dosages all work the same way and last about hours, with mg being an effective dosage for men who don’t respond to lower doses. What’s the correct dosage of Sildenafil (Viagra)? 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.