CHICAGO (Reuters) - Healthy men who took Cialis or Viagra daily for six months did not develop any significant vision problems, U.S. researchers said on Monday in a company-funded study aimed at addressing concerns about the popular impotence treatments.

Pfizer Inc's Viagra or sildenafil and Eli Lilly and Co's Cialis or tadalafil treat impotence by increasing blood flow to the genitals, but some men taking the drugs have reported blurred vision, blue-tinged vision or altered light perception.

The drugs work by blocking the action of the enzyme phodiesterase type 5 or PDE5 on the blood vessels in the penis. But PDE5 inhibitors may also act on blood vessels in the retina, which could explain some of the reports of vision problems.

To test this, a team led by Dr. William Cordell of Lilly Research Laboratories in Indianapolis and colleagues conducted a randomized study to look for changes in the retina among men taking Viagra or Cialis.

They studied 244 healthy men or those with mild erectile dysfunction aged 30 to 65 who had no vision problems. Of these, 85 took 5 milligrams of Cialis, 77 took 50 milligrams of sildenafil and 82 took a dummy pill daily for six months.

The men underwent comprehensive eye exams, including electroretinography, a test to measure the electrical response of the light-sensitive cells in the eye, before, during and after treatment.

Among the 194 men who completed the study, the researchers found no significant differences between treatment and placebo groups on electroretinography, visual function tests, measurements of pressure within the eyeball or assessments of the anatomy of the eye.

"Our results indicate that there is no cumulative damage or effect of clinical significance," for the studied doses of either drug, the researchers wrote in the Archives of Ophthalmology.

They said their study was limited because the doses used were lower than some other studies that did find retinal changes. And they said the results could not be generalized to men with eye problems, who were excluded from the study.

Viagra and Cialis each had global sales of more than $1 billion in 2008.

Eli Lilly designed and paid for the study, which did not evaluate Bayer AG's Levitra or vardenafil, a similar drug.

(Reporting by Julie Steenhuysen; Editing by Michael Conlon and Eric Walsh)

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