An array of "magnet therapy" products with claims of relieving various aches and pains reap millions in sales every year. However, a new study finds no evidence to support the notion that one of these products -- magnetic shoe insoles -- can ease foot discomfort.

That doesn't mean the insoles might not work via the "placebo" effect, however: According to the study, people who strongly believed in these products did experience pain relief -- even if they were given false, nonmagnetic insoles.

The randomized, double-blind, placebo-controlled study "provides convincing evidence that use of these static magnets was not effective in relieving symptoms of nonspecific pain in the workplace," lead researcher and Mayo Clinic physician Dr. Mark Winemiller said in a prepared statement. His team published its findings in the current issue of Mayo Clinic Proceedings.

He noted that many adults with foot pain are likely to use magnets based on their personal beliefs or recommendations from other people. Worldwide, people spend about $5 billion a year on different kinds of magnetic pain-relieving devices, which have never been approved for therapeutic use by the U.S. Food and Drug Administration.

The insoles used in this study were the Active Comfort magnetic insole made by Spenco Medical Corp., of Waco, Texas. These insoles have a magnetic foil pad located under the arch of the foot.

The study was funded by an unrestricted grant from Spenco, which played no role in the study design or in the researchers' interpretation of the data.