U.S. health regulators on Friday said they have received more than 40 reports of a type of Blindness in men taking impotence drugs, mostly involving Pfizer Inc.‘s Viagra.

The U.S. Food and Drug Administration said it has received about 38 reports of the rare condition among users of Viagra, four reports of blindness among users of Eli Lilly and Co.‘s Cialis and one report of the condition in men talking Levitra, made by GlaxoSmithKline Plc.

Pfizer said outside of clinical trials, Viagra has been used by more than 23 million men worldwide over the past seven years. It said reports are extremely rare of visual loss due to the condition known as non-arteritic anterior ischemic optic neuropathy (NAION).

“FDA is aware of these reports but has not determined there is a cause and effect due to use of Viagra ... We’re working with the company to make sure this information is available to doctors and patients,” said FDA spokeswoman Susan Cruzan.

New York-based Pfizer acknowledged the rare cases of blindness and is weighing a label change. But it emphasized no proof exists that the blindness is linked to the drug, which was introduced in 1998 and had $1.68 billion in sales in 2004.

“We are in discussions with the FDA to update our language to reflect these rare ocular events that have occurred,” Pfizer spokesman Daniel Watts said.

Viagra’s label currently lists the most common side effects as headache, facial flushing and upset stomach, and says the less common are bluish or blurred visions and sensitivity to light that may occur for a short time.

“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,” Pfizer said in a statement.

The FDA’s Cruzan said NAOIN is not uncommon for people of the age that use the impotence drugs, who have conditions like heart conditions, Diabetes and Heart disease. It also occurs in patients who have not taken Viagra.

“FDA continues to evaluate the situation and recommends that patients talk to their health-care provider if they have specific questions,” Cruzan said.

Officials for GlaxoSmithKline and Lilly were not immediately available. Cruzan said the FDA “would be talking to them (the manufacturer) about adding that information to the label.”

Pfizer shares fell 55 cents, or 1.9 percent, to $28.35 in morning trade on the New York Stock Exchange. Shares of Eli Lilly & Co. which makes rival impotence drug Cialis, rose 3 cents to $59.13 on the NYSE. GlaxoSmithKline Plc, which makes Levitra rose 31 cents to $49.71 on the NYSE.