Generic versions of sildenafil, vardenafil and tadalafil are available. Talk to your doctor about eating grapefruit and drinking grapefruit juice while taking this medicine. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily. What special precautions should I follow? Its effects often last for 4 to 5 hours. How oral medicines differ Each oral medicine for ED has a slightly different chemical makeup. This can happen if you are sick with fever, diarrhea, or vomiting; sweat a lot; or do not drink enough liquids.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 . Viagra is available in mg, mg, and mg doses and can be purchased in day and day supplies. Shipping is free, and your new medication can be sent . Still, if your doctor says it’s okay for you to use and you end up paying out-of-pocket, you can still get the generic for a lot less than Viagra. According to price-comparison site .