Hughes 1991.

Study characteristics
Methods	Study design: parallel RCT Country: USA Recruitment: primary care patients
Participants	106 smokers, motivation to quit not required 48% men, average age 38, average cigarettes per day 26
Interventions	1) Free prescription for nicotine gum for up to 6 months 2) Nicotine gum at cost of USD 6/box (96 pieces 2 mg) 3) Nicotine gum at USD 20/box All participants received brief physician advice with 1 follow‐up
Outcomes	Abstinence at 6 months Validation: observer verification of all 6‐month quitters Adverse events: not measured
Notes	Tested effect of price on gum use and efficacy. We combined groups 2 and 3 to make 1 purchasing arm in meta‐analysis. Similar quit rates in the 2 combined arms This study was supported by a grant (DA‐04066) and Research Scientist Development Award (DA‐00109) from the National Institute on Drug Abuse. Merrell‐Dow Research Institute provided nicotine gum. Conflicts of interest: not reported
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Not stated
Allocation concealment (selection bias)	Low risk	Quote: "Physician opened a sealed envelope" which assigned participants to a price group
Blinding (performance bias and detection bias) All outcomes	High risk	Double‐blind, as described above. But physicians knew how much each participant paid, and therefore which group they were in, so could have managed them differently (Quote: "no anecdotal evidence that this occurred")
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	Losses at 6 months reported; all were counted as failures, but distribution across the groups not reported