Exploration of the validity of weak magnets as a suitable placebo in trials of magnetic therapy.

OBJECTIVE: To investigate whether 50 mT magnetic bracelets would be suitable as a placebo control condition for studying the pain relieving effects of higher strength magnetic bracelets in arthritis. DESIGN: Randomised controlled comparison between groups given either a weak 50 mT or a higher strength 180 mT magnetic bracelets to test. SETTING: Four arthritis support groups in Devon, UK. PARTICIPANTS: One hundred sixteen people with osteoarthritis and rheumatoid arthritis. MAIN OUTCOME MEASURE: Beliefs about group allocation and expectation of benefit. RESULTS: There was no significant difference between groups in beliefs about allocation to the 'active magnet' group. Participants were however more likely to have an expectation of benefit (pain relief) with the higher strength magnetic bracelets. CONCLUSIONS: Asking about perceived group allocation is not sufficient to rule out placebo effects in trials of magnetic bracelets which use weak magnets as a control condition. There are differences in expectation of benefit between different magnet strengths.