Pfizer shares fell 1.9% on Friday after the news broke. The report may turn out to be inconclusive since the type of blindness found is also associated with men with diabetes and heart disease, conditions that also cause erectile dysfunction.

Both the agency and Pfizer said, however, that it remains unclear whether the drug was actually associated with the blindness. In a statement, Pfizer said that a review of 103 clinical trials with Viagra involving 13,000 patients found no reports of the sudden blindness, known as non-arteritic anterior ischemic optic neuropathy (NAION). "There is no evidence showing that NAION occurred more frequently in men taking Viagra than men of similar age and health who did not take Viagra," the statement said.

FDA spokeswoman Suzanne Trevino said the FDA "has not determined that there is a cause and effect due to these drugs." She added: "We are working with the company to ensure that this information is available to doctors and patients."

More than 23 million men worldwide have taken Viagra over the past seven years, Pfizer said