Children 1 year of age and older weighing 20 to 45 kg—20 mg 3 times a day. Sildenafil usually should not be taken more than once every 24 hours. Sildenafil treats erectile dysfunction by increasing blood flow to the penis during sexual stimulation. Avanafil Stendra. Sildenafil Viagra is used to treat erectile dysfunction impotence; inability to get or keep an erection in men.Take sildenafil as needed before sexual activity. The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 . The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called . Overdose Ask your doctor Q&A Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a .