Be sure to let your doctor and pharmacist know that you are taking these medications before you start taking sildenafil. Your doctor may need to change the doses of your medications or monitor you carefully for side effects. If you are taking sildenafil to treat erectile dysfunction, you should know that it does not cure erectile dysfunction or increase sexual desire. Sudden vision loss in one or both eyes can be a sign of a serious eye problem called non-arteritic anterior ischemic optic neuropathy NAION. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily. Sildenafil usually should not be taken more than once every 24 hours.Why is this medication prescribed? Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. 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 . SAVINGS Terms and conditions apply. take any medicines called nitrates, often prescribed for chest pain, or guanylate cyclase stimulators like Adempas (riociguat) for pulmonary .