U.S. researchers have found vision impairment in some patients who use Viagra (sildenafil) for erectile dysfunction. A new study in the Journal of Neuro-Opthamology describes seven people developing nonarteritic ischemic optic neuropath (NAION), an eye ailment that sometimes results in blindness. Including other studies, as many as fourteen people have been affected by NAION after using Viagra. Doctors say vision loss is among the most serious side effects exhibited by patients, but is rare. Most patients exhibit signs such a blurred vision within 24-hours. Researchers say all of the patients had additional risk factors, including her disease and high blood pressure. Many also had high cholesterol levels as well, while others had preexisting eye conditions. Researchers say it's still to soon to acknowledge a cause and effect relationship between sildenafil and NAION, but patients should be cautious.