Also tell your doctor if you or any of your family members have or have ever had an eye disease such as retinitis pigmentosa an inherited eye condition that causes loss of vision or if you have ever had sudden severe vision loss, especially if you were told that the vision loss was caused by a blockage of blood flow to the nerves that help you see. Hypertension: High blood pressure also damages blood vessels. Sildenafil may cause side effects. Vardenafil usually works for 4 to 5 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.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 alcohol. Its . Your doctor may start you on a lower Viagra dosage than usual if you: are age 65 years and older have liver problems, such as cirrhosis (scarring of the liver) have severe kidney . Viagra is approved for this use in males ages 18 years and older. Viagra contains the drug sildenafil. It belongs to a class of drugs called phosphodiesterase type 5 (PDE5) .