It continues to work for up to 4 hours, although its action is usually less after 2 hours. The four main medicines taken by mouth for erectile dysfunction are: Avanafil Stendra. Am J Psychiatry. If your dose is different, do not change it unless your doctor tells you to do so. Adults 65 years of age and older—25 mg as a single dose no more than once a day, 1 hour before sexual intercourse.Why is this medication prescribed? Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men.  · Sildenafil (Oral Route) Description and Brand Names. Drug information provided by: Merative, Micromedex ®. US Brand Name. Revatio. Viagra. Descriptions. . Oral medicines for ED usually work well and cause few side effects. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil .