How should this medicine be used? Your doctor may need to change the doses of your medications or monitor you carefully for side effects. Tell your doctor if any of these symptoms are severe or do not go away: headache. This site is intended only for U. Sexual activity may be a strain on your heart, especially if you have heart disease. What should I do if I forget a dose?Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in . If you are taking sildenafil (Viagra) for ED, do not take it more than once a day. Take an hour before sexual stimulation (range 30 minutes to 4 hours). provides . The Savings Offer and Program expire on 12/31/ For further information, call , visit , or write: Viatris Inc., P.O. Box , Mission, KS .