In sufficient doses it could cause damage to the retina of the eye, the New Scientist magazine reports today.
The erection pill has taken America by storm. Less than three weeks after its launch, doctors in the United States had written an unprecedented 113,000 prescriptions for Viagra.
The pill even received an endorsement from the former presidential candidate Bob Dole, and the drug looks set to have the same effect in Britain.
But it is known to have a side-effect that causes "blue vision" and some eye experts fear that Viagra may do more than that, even in sufficient doses damaging the retina. Viagra works by inhibiting an enzyme called phosphodiesterase, which allows more blood to flow into the penis.
A similar enzyme exists in the cone cells responsible for colour vision in the retina. It is the effect on this enzyme that leads to a perception of the colour blue, and which is concerning eye specialists.
The drug is thought to mimic a condition where levels of phosphodiesterase are abnormal.
"This has ophthalmologists worried, because people with congenitally abnormal phosphodiesterase suffer irreversible damage to their retinas over time," said the report in the New Scientist.
The American Academy of Ophthalmologists is now pressing Pfizer, the drug company which manufactures Viagra, to conduct more studies. Michael Marmor, an eye specialist at Stanford University in California, told the magazine: "The company has not measured the electrical activity of the cone cells in the long term."
But Pfizer said that rigorous tests at doses well above the recommended level had shown no clinically significant effect on vision in either the short term, or the long term.
There are also already fears that some men wanting to reclaim youthful vigour are taking higher than recommended doses despite their doctors' warnings.
In the clinical trials which convinced the American Food and Drug Administration to approve Viagra, the "magic bullet" drug quadrupled men's success at having sexual intercourse and significantly improved the quality of their erections. In one study, men receiving 100 milligrams of Viagra, the highest recommended dose, doubled their frequency of erections.
But one sexual health expert told New Scientist that the reality of Viagra's performance did not match the hype.
However, James Barada, of the Center for Male Sexual Health in Albany, New York, said that many of the men in the trials had achieved intercourse in the previous three months and so were not clinically impotent. He claimed that only one-third of men who were genuinely impotent, were able to have intercourse after taking the drug.
Mr Barada, who has written more than 150 prescriptions for Viagra, said: "I consider them Viagra failures. You don't take a drug just because it helps a little bit."