"The agency is advising consumers to stop taking [Viagra, Cialis and Levitra] and call a doctor or health-care provider immediately if they experience sudden or decreased vision loss in one or both eyes, a condition known as NAION (non arteritic ischemic optic neuropathy) that occurs when blood flow is blocked to the optic nerve. Anyone taking or considering taking these erectile dysfunction drugs should also tell their health-care provider if they've ever had severe loss of vision, which might reflect a prior episode of NAION. Such patients are at an increased risk of developing NAION again, the FDA said in a statement." (reference)