US health officials on Friday launched an investigation into reports of blindness among some men using the impotence drug Viagra.

This type of blindness, called onon-arteritic anterior ischemic optic neuropathy (NAION), can occur in men who are diabetic or have heart disease, the same conditions that can cause impotence and thus lead to Viagra use.

The FDA has received 38 complaints of blindness among Viagra users, according to a CBS News report. The drug for erectile dysfunction has been taken by more than 23 million men worldwide.

"We take this seriously," said FDA spokeswoman Susan Cruzan.

The Food and Drug Administration (FDA) said it has asked Pfizer Inc., the world's largest drug maker that produces Viagra, to amend warnings on label although no evidence has been found so far that the drug is to be blamed.

Pfizer issued a statement on its website, saying it was in discussions with the FDA to "update the Viagra label to reflect these rare ocular occurrences."

The statement said there were no reports of NAION during its clinical trials of Viagra involving 13,000 patients.

"NAION is the most common acute optic nerve disease in adults over age 50 and it shares a number of common risk factors with erectile dysfunction: age over 50, high blood pressure, high cholesterol, diabetes," it said.

"Most of the reported cases in which NAION has occurred in men taking Viagra have involved patients with underlying anatomic or vascular risk factors associated with the development of NAION," it added.

Viagra was approved by the US government in 1998. It may aid in the treatment of enlarged hearts that can result from high blood pressure, tests on animals indicate.

Source: Xinhua