Since its launch in 1998, Viagra, the world's first treatment for impotency, has revolutionized life for men of a certain age: more than 23 million worldwide have used the blue, diamond- shaped pills to spice up their love lives.

Starting to Spring of 2005 American health officials are examining reports that it causes blindness among a small number of at-risk men. The Viagra blindness risk was also reported in March 2005 in the Journal of Neuro-Ophthalmology. The journal study examined seven cases where men suffered an unusual form of sudden blindness within 36 hours of taking Viagra.

Pfizer spokesperson Daniel Watts said there is no proof that Viagra caused the blindness. He said that men who take Viagra often have high blood pressure and high cholesterol, which are also associated with the conditions that can causes blindness. This form of sudden blindness is caused by the blockage of blood flow to the optic nerve and is most common in older people, who disproportionately use Viagra.

All these patients had other risk factors for this type of vision loss, including high blood pressure, high cholesterol or diabetes.

As researchers remain uncertain, FDA officials said they did not know if the drugs cause the blindness. But they ordered that labels for erectile dysfunction medications must include warnings about a potential link with sudden blindness. The new label said such vision loss, known as non-arteritic anterior ischemic optic neuropathy, "has been reported rarely."

Canadian ministry also advised users of the erectile dysfunction drugs Viagra, Cialis or Levitra to seek "immediate medical attention" if they experience sudden vision loss or other vision problems when taking the drugs.