Two hugely popular supplements used to fight arthritis and joint pain, glucosamine and chondroitin sulfate, do not seem to work any better than placebo to slow the loss of knee cartilage in osteoarthritis, researchers reported.
But the researchers said some of their findings were confusing and said more study was needed.
"At two years, no treatment showed what we determined to be a clinically important reduction in joint space width loss," said Dr. Allen Sawitzke of the University of Utah School of Medicine, who helped lead the study.
The study, funded by the National Center for Complementary and Alternative Medicine at the National Institutes of Health, confirms other findings showing the supplements have few or no effects.
The trial is called glucosamine/chondroitin arthritis intervention trial or GAIT. Writing in the October issue of Arthritis & Rheumatism, Sawitzke and colleagues said Monday they had trouble interpreting their results because patients who took placebos had a smaller loss of cartilage than they should have.
The original GAIT study results in 2006 found the supplements did not reduce the pain of knee arthritis, except among a small group of patients with moderate to severe pain.
The GAIT researchers continued to watch 572 volunteers for another 18 months and found the supplements did not appear to slow the loss of cartilage, taken either alone or together.
They said arthritis worsened in 24 percent of participants taking both, similar to those taking placebo.
"Research continues to reveal that osteoarthritis, the most common form of arthritis, appears to be the result of an array of factors including age, gender, genetics, obesity, and joint injuries," said Dr. Stephen Katz, director of the NIH's National Institute of Arthritis and Musculoskeletal and Skin Diseases.
"Because osteoarthritis affects nearly 21 million Americans, we are seeking ways to not only treat pain, but also address the structural effects of the condition," he said.