Click here for an email preview. It works for up to 36 hours. Kidney disease that needs to be treated with dialysis. Push the plunger of the syringe, then insert the tip into the bottle while holding the bottle upright on a flat surface. Sildenafil Viagra. Talk to your doctor about eating grapefruit and drinking grapefruit juice while taking this medicine.It’s a prescription drug that’s used in adult males* with ED. With ED, you have trouble getting or keeping an erection. Viagra treats ED by increasing the amount .  · For oral dosage form (tablets): Adults up to 65 years of age—50 milligrams (mg) as a single dose no more than once a day, 1 hour before sexual intercourse. . 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 .