FRIDAY, Jan. 30 (HealthDay News) -- Eager to relieve joint pain and repair the cushioning between bones, millions of arthritis sufferers reach for glucosamine, an over-the-counter dietary supplement.

Despite its popularity, studies examining the effectiveness of this natural therapy have yielded mixed results.

"There is still a lot of uncertainty about glucosamine," said Dr. Steven C. Vlad, a fellow in clinical epidemiology and rheumatology at Boston University School of Medicine.

So what is glucosamine, anyway? It's a type of sugar that the body produces and distributes in cartilage and other connective tissue. Chondroitin sulfate, often taken in combination with glucosamine, is a complex carbohydrate that helps cartilage retain water, according to the U.S. National Center for Complementary and Alternative Medicine.

These substances are derived from animal tissues, according to the Arthritis Foundation. Glucosamine is extracted from crab, lobster or shrimp shells, and chondroitin sulfate comes from animal cartilage, such as tracheas or shark cartilage.

Vlad's own study tried to determine why the results of glucosamine trials differed so widely. Of the 15 studies he reviewed, there was one clear finding: A particular glucosamine preparation, called glucosamine hydrochloride, doesn't work.

Results among trials involving another common preparation, glucosamine sulfate, showed wide variation -- more than would be expected by chance. Based on the evidence, Vlad concluded that supplement industry support could be a source of bias in some of these studies.

Although critics questioned Vlad's finding, he stands by the results. "Numerous analyses have showed that industry funding is correlated with stronger findings and selective publication of positive results," he noted.

More recently, researchers at the University of Utah School of Medicine reported results from a follow-up to an earlier glucosamine trial. Arthritis patients in this leg of the study took glucosamine; a combination of glucosamine and chondroitin; the anti-inflammatory drug celecoxib (Celebrex), or a placebo. X-rays were taken of patients' knees before the trial began and one and two years later to determine whether glucosamine alone, or in combination with chondroitin, would slow the loss of cartilage.