Intended for healthcare professionals

Endgames Statistical Question

Measuring the detriment of treatment: number needed to harm

BMJ 2015; 350 doi: (Published 22 May 2015) Cite this as: BMJ 2015;350:h2763
  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Institute for Medical and Biomedical Education, St George’s, University of London, London, UK
  1. p.sedgwick{at}

Researchers investigated the efficacy and safety of nicotine patches in pregnant women who smoked. A randomised placebo controlled trial was performed. The intervention consisted of the administration of 16 hour nicotine patches until the time of delivery. Participants were pregnant women over 18 years who smoked at least five cigarettes a day and whose babies were between 12 and 20 weeks’ gestation. In total, 402 women were recruited from 23 maternity wards throughout France and randomly allocated to the intervention (n=203) or placebo patches (n=199).1

The primary outcome measures were achievement of complete abstinence until delivery and birth weight. The proportion of women who achieved complete abstinence was higher in the nicotine patch group than in the placebo group, although the difference was not significant (5.5% (n=11) v 5.1% (n=10); P=0.87). The mean birth weight was higher in the nicotine patch group, although the difference was not significant (3065 (standard error 44 g) v 3015 g (44 g); P=0.41). The incidence of serious adverse events (including stillbirth, late miscarriage, and newborn death at birth) was similar between the treatment groups. Non-serious adverse reactions, mainly affecting the skin, were more common in the nicotine patch group than in the placebo patch group. In particular, skin reactions at the patch site were reported by 23 (11.33%) women in the intervention group compared with 8 (4.02%) in the placebo patch group (number need to harm 13.7; 95% confidence interval 8.0 to 46.1). It was concluded that the nicotine patch did not increase the smoking cessation rate or birth weight, although it increased the frequency of non-serious adverse reactions, mainly those affecting the skin.

Which of the following statements, if any, are true?

  • a) It is estimated that, on average, for every 13.7 pregnant women who used nicotine patches one would experience an …

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