News for millions of men who have benefited from Viagra since it was introduced in 1998: A recent study suggests that the power of the blue anti-impotence pill might decline over time for some people. Urologist Rizk El-Galley of the University of Alabama at Birmingham surveyed a small sample—82 patients—two years after they had first filled prescriptions for Viagra. Among the group, 43 men, or 52 percent, were still taking the drug. Of those, 16 had doubled the dose. Fourteen of the men who had discontinued use said the drug was no longer effective. El-Galley says that the body may compensate for the sudden surge of Viagra in the bloodstream by reducing the number of receptors that respond to it, thus building up tolerance. But urologist Mike Sweeney of Pfizer, which manufactures Viagra, says the company's studies indicate that only 5 percent of users cease taking the drug because of lack of efficacy. He suggests the pill may stop working not because of drug tolerance but because the underlying diseases that cause erectile dysfunction, such as diabetes and hypertension, may worsen over time. Urologist Tom Lue of the University of California at San Francisco agrees: Although other medications that dilate blood vessels—such as daily antihypertensive drugs—do induce tolerance, such an outcome seems unlikely from sporadic doses of Viagra. He is more concerned about a new Viagra-like drug, due out later this year, that remains active at least six times as long: "If you have a medication that lasts more than 24 hours, the body may want to compensate."