Despite worries that pills to treat erectile dysfunction can harm vision, a new study of two drugs found no side effects after six months of daily use.

The fears came about because the drugs affect how a compound known as PDE5 works on blood vessels in the penis. The compound is also active in the retina, and some users of the drugs reported mildly blurry vision, blue-tinged vision or changes in light.

To test the problem, researchers from drug maker Eli Lilly found 244 healthy men, some with mild erectile dysfunction. They were randomly assigned to take either sildenafil, sold as Viagra; tadalafil, sold as Cialis; or a placebo for six months.

They were also given eye tests, including test of electrical activity in the eye.

Researchers said in a news release that 194 finished the study, and there were no significant differences between those who took drugs and those who had the placebo.

"Our results indicate that there is no cumulative damage or effect of clinical significance for either 5 milligrams of tadalafil or 50 milligrams of sildenafil taken daily for six months," the authors said.

The report appeared in the April issue of Archives of Ophthalmology.