Will Viagra make me instantly erect? Sorry something went wrong with your subscription Please, try again in a couple of minutes Retry. When side effects do occur , these are generally mild and short-lived. Your doctor may adjust your dose as needed and tolerated. Avoiding antacids: Eating a light meal will help out with this one! Adults 65 years of age and older—25 mg as a single dose no more than once a day, 1 hour before sexual intercourse. Click here for an email preview.Viagra works at its best within a few hours after it’s taken. In most people, the drug is most effective about 1 hour after it’s taken. Over time, Viagra is slowly .  · Adults 65 years of age and older—25 mg as a single dose no more than once a day, 1 hour before sexual intercourse. Alternatively, the medicine may be .  · Viagra is usually taken only when needed, 30 minutes to 1 hour before sexual activity. You may take it up to 4 hours before sexual activity. Do not take two doses at .