How Taken Viagra should be taken as needed about 1 hour before sexual activity. However, Viagra can be taken anytime from 4 hours to 30 minutes before sexual activity. Drug Class and Mechanism Viagra is a phosphodiesterase inhibitor. It works by helping the blood flow into the penis to achieve and maintain an erection. Missed Dose Viagra is used as needed, so you are not likely to miss a dose. If you miss a dose of Viagra, take the missed dose as soon as you remember. If it is almost time for your next dose, skip the missed dose and take the medicine at the next regularly scheduled time. Do not take extra medicine to make up the missed dose. Storage Keep Viagra in its original container. Store at 25 degrees C (77 degrees F);