A Cochrane review included 21 studies. The odds ratio (OR) for quitting with incentives at longest follow-up (6 months or more) compared with controls was 1.42 (95% CI 1.19 to 1.69; 17 trials, n=7 715). Only 3 studies demonstrated significantly higher quit rates for the incentives group than for the control group at or beyond the six-month assessment. Deposit-refund trials suffered from relatively low rates of uptake, but those who did sign up and contributed their own money achieved higher quit rates than reward-only participants. Eight of nine trials with usable data in pregnant smokers delivered an adjusted OR at longest follow-up (up to 24 weeks post-partum) of 3.60 (95% CI 2.39 to 5.43; 1295 participants, moderate-quality studies) in favour of incentives.
Comment: The quality of evidence is downgraded by limitations in study quality, by inconsistency (unexplained variability in results), anad by imprecise results (few patients for each comparison).