Do not take a double dose to make up for a missed one. Please expect up to 4 to 6 weeks for reimbursement. These include a heart attack, stroke or a serious heart rhythm disorder in the past six months. By controlling the enzyme, sildenafil helps to maintain an erection after the penis is stroked. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand.Ask your doctor to prescribe brand-name VIAGRA and to indicate “DAW” (Dispense As Written)—or the language used in your state—on all of your VIAGRA prescriptions. Get .  · Sildenafil is used to treat men who have erectile dysfunction (also called sexual impotence). Sildenafil belongs to a group of medicines called phosphodiesterase . Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men.