Chicago, May 27 (Reuters): Pfizer Inc. today acknowledged rare cases of blindness in men taking its impotence drug Viagra and said it is in talks with US regulators to change the drug’s label.

Viagra, introduced in 1998, had $1.68 billion in sales in 2004. Reports of blindness have surfaced before, and Pfizer ? the world’s largest drugmaker ? emphasised that no proof exists that the blindness is linked to the drug.

“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.