Effect of placebo groups on blood pressure in hypertension: a meta-analysis of beta-blocker trials

Hypertension is often treated pharmacologically. Since there is evidence that the cardiovascular system is sensitive to placebo mechanisms, our aim was to conduct an effect size analysis of placebo groups in double-blinded randomized controlled parallel-group drug trials using beta-blockers to treat hypertensive patients. A comprehensive literature search via PubMed, PsycINFO, PSYNDEX, PQDT OPEN, OpenGREY, ISI Web of Knowledge, and the WHO International Clinical Trials Registry Platform provided the basis of our meta-analysis. Effect sizes were estimated using a random-effects model based on 23 studies covering a total of 11,067 participants. Main outcomes were systolic blood pressure (sBP) and diastolic blood pressure (dBP). Blood pressure was lowered in placebo groups with significant and robust effect sizes (Hedges' g). The estimates for sBP (−0.27, P < .001) and dBP (−0.49, P < .001) can be interpreted as small to moderate. The placebo response accounted for 34% of the drug response for sBP and 47% of the drug response for dBP. Our moderator analyses indicated that a higher study quality and more study site visits were marginally associated with a higher placebo response. In light of these strong placebo responses, placebo mechanisms need to be considered in order to improve antihypertensive treatment.