Shake the liquid well for 10 seconds before each use to mix the medication evenly. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily. If you are taking sildenafil for erectile dysfunction, your doctor will probably start you on an average dose of sildenafil and increase or decrease your dose depending on your response to the medication. Sexual activity may be a strain on your heart, especially if you have heart disease. Before starting, stopping, or changing any medications while taking sildenafil, please get the advice of your doctor or pharmacist.Ask your doctor. Q&A. Drug Images. What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription drug that’s used in . Sexual Health. Birth control. STIs. HIV. Activity. Relationships. Are There Benefits to Taking Viagra Daily? Can you? While you sleep. Benefits. How it’s used. Side effects. Other . Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). The drug comes as a tablet you’ll take by swallowing. It’s usually taken as needed about 1 .