Purpose: To determine whether newly published evidence of inefficacy of dietary supplements is associated with a decline in supplement sales trends after study publication.

Methods: Using market data representing major grocery stores, drug stores, and mass merchandisers (10% of total US market), we analyzed sales of supplements in the months before and after the release of results for the four major clinical trials of dietary supplements published between January 2005 and December 2006: echinacea (July 2005), saw palmetto (February 2006), black cohosh (December 2006), and glucosamine/chondroitin (February 2006). Primary outcomes for each of these trials showed no benefit compared to placebo controls. (A planned secondary analysis in the glucosamine/chondroitin study showed efficacy.) For each supplement trial, we calculated mean 4-week sales for (7, 4, 14, and 14 4-week cycles respectively) before and after publication of results, depending on available data. We performed unpaired two sided t-tests assuming unequal variances (alpha = .05) to compare pre- and post-publication mean sales for each supplement to test observed differences in mean sales. We used seasonally comparable time periods pre and post-trial for echinacea.

Results: Total supplement sales were constant or slightly increased for the period January 2005 to December 2006 ($US2.27 -2.28 billion annually). Sales for 3 of the 4 supplements showed a decline in sales after publication of results. Based on 14 4-week cycles of sales pre and post publication, saw palmetto sales decreased 13.6 % from $1.5US million/4-weeks to $1.32US million/4-weeks (p=<.001); for 7, 4-week cycles from seasonally identical periods pre and post publication, echinacea sales decreased 17% from $1.4US million/4-weeks to $1.2 million/4-weeks (p=.08) and black cohosh sales decreased 22% from $ .73US million/4-weeks to $.60US million/4-weeks based on 4, 4-week cycles of data (-22%; p=.06 ). Sales for glucosamine/chondroitin increased slightly (1.3%) from $US22.5 million/4-weeks to $US22.8 million/4-weeks (p=.50) based on 14, 4-week cycles.

Conclusion: Downward changes in individual sales of dietary supplements seem to be temporally associated with publication of new inefficacy data for dietary supplements. These unadjusted analyses cannot establish causal relationships. Future research should identify factors that enhance the impact of new evidence on consumers' decisions to purchase and use dietary supplements.