HEALTH officials in the US and UK are examining rare reports of blindness among some men using the impotence drug Viagra.
The American Food and Drug Administration (FDA) still is investigating, but has no evidence yet that the drug is to blame, said spokeswoman Susan Cruzan.
The type of blindness is called NAION, or non-arteritic anterior ischemic optic neuropathy.
It 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 50 reports of the blindness. Viagra has been taken by more than 23 million men worldwide. "We take this seriously," added Ms Cruzan.
In the UK, the authorities said they had been notified of three "suspected adverse drug reaction" reports of blindness, one temporary.
On its website, Viagra manufacturer Pfizer Inc stated: "The most common side-effects of Viagra are headache, facial flushing, and upset stomach.
"Less common are bluish or blurred vision, or being sensitive to light. These may occur for a short time."
This statement had been available before the current inquiry and mirrors advice given by the UK equivalent of the FDA, which has conducted an investigation into Viagra on the same grounds and found no supporting evidence.
The FDA is working with Pfizer to determine what, if any, information about the condition should be added to the drug's label.
Viagra, approved to treat erectile dysfunction, should not be used by men with heart conditions whose doctors have warned them not to have sex. Also, patients taking drugs that contain nitrates have been warned not to take Viagra because of sudden, unsafe drops in blood pressure.
The drug's label also warns of painful erections lasting longer than six hours, headache, flushed skin and vision problems.
A spokesman for the UK equivalent of the FDA, the Medicine and Healthcare Products Regulatory Agency, said: "Since Viagra has been authorised in Europe, it has been extensively used in the UK. In this context, [we have] received a total of three suspected adverse drug reaction (ADR) reports of blindness, including one temporary blindness.
"In two of these three cases the blindness was associated with optic neuropathy. Where information was provided, all patients had underlying illnesses including high blood pressure which are known risk factors for eye disorders.
"It is important to note that a report of a suspected adverse reaction does not necessarily mean that it was caused by the drug and may relate to other factors such as concurrent illnesses, as in the three UK reports, or medicines taken concurrently."
