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The results of two 6-month long, randomized, double-blind, controlled trials (RCT) of the anticonvulsant pregabalin [5] and the balanced serotonin–norepinephrine re-uptake inhibitor (SNRI) duloxetine [13] are published in this issue of PAIN. While these studies were designed to generate safety and efficacy data for two new fibromyalgia (FM) medications they used very different trial designs to achieve these goals. In the duloxetine study effectiveness for FM symptoms was measured through improvement of primary and secondary endpoints, including pain. In contrast, the pregabalin trial used group membership (P30% pain reduction) after 6 months as a surrogate measure of effectiveness. In this editorial we propose that factors contributing to the placebo effect likely affected each trial differently. Medication side effects could easily have biased each trial through cues that signaled the presence of the active drug condition. In the duloxetine study, placebo effects may have been minimized by terminating patients with early adverse events/side effects or inflated by the remaining difference in side-effect profiles between the two groups. However, because of its unusual enrichment design, the pregabalin study may not only have compromised its allocation concealment but also inflated the difference between pain levels across placebo and active treatment groups by factors now known to generate placebo effects [12].

Long-term trials of pregabalin and duloxetine for fibromyalgia symptoms: How study designs can affect placebo factors