It works for up to 36 hours. Oral medicine for ED also might not be safe or work well if you have: Very low blood pressure, called hypotension, or uncontrolled high blood pressure, called hypertension. Its effects often last for 4 to 5 hours. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. This can happen if you are sick with fever, diarrhea, or vomiting; sweat a lot; or do not drink enough liquids. You can take it in a small dose daily or in a larger dose as needed.Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or . Since , Viagra (sildenafil) has been used for treating erectile dysfunction (ED). It’s available by prescription for males ages 18 years and older. Side . 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 .