Updated from 9:32 a.m. EDT

The Food and Drug Administration is investigating reports that some men have lost their sight while using Viagra, the pill for treating impotence, according to published reports.

According to the

Associated Press

, which cited an FDA spokeswoman, the government is still investigating but doesn't have any evidence the

Pfizer

(PFE) - Get Report

drug is responsible.

The type of blindness being examined can occur in men who are diabetic or have heart disease, which can also lead to erectile dysfunction, the

AP

reported.

CBS News

was credited by

AP

with first reporting the story.

Pfizer told the

AP

that it has had talks with the FDA about including a statement on the drug's label to indicate men taking Viagra had developed blindness in rare instances. A company spokesman told the news organization there wasn't any proof Viagra caused the men to lose their sight.

Additionally, the FDA has reports of vision loss by four men who used Cialis, an ED drug made by

Eli Lilly

(LLY) - Get Report

and

ICOS

(ICOS)

, the

AP

reported.

The report indicated that the FDA has received 50 reports of the blindness. Viagra has been on the market since 1998, and more than 23 million men have taken the drug. Sales of Viagra were $438 million in the first quarter, Pfizer said last month.

The company's Viagra Web site says the most common side effects of the drug are headache, facial flushing and upset stomach, but that it may also briefly cause bluish or blurred vision or sensitivity to light.

Shares of Pfizer were down 63 cents to $28.27. Lilly lost 3 cents to $59.07, and ICOS shed 14 cents to $22.43.