A spokeswoman says the Food and Drug Administration is taking "seriously" reports of blindness among some men who use the impotence drugs Viagra and Cialis.

The agency says it has 42 reports of the blindness, 38 among users of Viagra and four among Cialis users.

The agency says it's investigating the blindness claims, but has no evidence yet the drugs are to blame.

It's a type of blindness that can occur in men who are diabetic or have heart disease. Those same conditions can cause impotence.

More than 23 million men have taken Viagra worldwide.

Its maker, Pfizer says on its Web site that Viagra's most common side effects "are headache, facial flushing, and upset stomach."

It says vision problems are "less common."

Eli Lilly, which makes Cialis, also refers on its Web site to vision problems as an uncommon side effect of the drug.

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