Intended for healthcare professionals

Endgames Statistical Question

The placebo effect

BMJ 2011; 343 doi: (Published 30 November 2011) Cite this as: BMJ 2011;343:d7665
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}

Varenicline is used as a smoking cessation aid. Its efficacy and safety were assessed by a double blind, placebo controlled, randomised controlled trial. In total, 213 participants were randomised to varenicline and 218 to placebo. Treatment was for 12 weeks, and participants were followed up for 14 weeks after treatment.1

The primary end point was continuous abstinence from smoking for the final four weeks of treatment (weeks 9 to 12). The observed treatment effect—the rate of continuous abstinence—was 59% in the varenicline group and 39% in the placebo group. The difference between the varenicline and placebo treatment groups in continued absence between weeks 9 and 12 was significant (relative risk 1.6 (95% …

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