CHICAGO (Reuters) – Healthy men who took Cialis or Viagra daily for six months did not develop any significant vision problems, US researchers said last week 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.