Walker et al. BMC Public Health 2011, 11:37 http://www.biomedcentral.com/1471-2458/11/37 Randomisation: allocation concealment and sequence generation All potential participants will be assigned a unique regis- tration number allocated by a central computer following details submitted on a web-based form. This number will be used to identify each randomised participant who has consented to take part. Participants will be randomised by computer, with stratified minimisation by sex, ethni- city (Māori, non-Māori), and level of nicotine depen- dence (> 5 points, ≤ 5 points on the Fagerström Test of Nicotine Dependence (FTND) Questionnaire [6]), to ensure a balance in these key prognostic indicators between the intervention and control group. Blinding Due to the nature of the intervention only single blinding (of researchers, not of participants) is possible in this trial. Members of the trial steering committee, management committee, and other team members from the Clinical Trials Research Unit (with the exception of the project co- ordinator and the Quitline research manager) will remain blinded to treatment allocation until the code is broken (after the last follow-up call is completed and the data recorded). The project co-ordinator will not be blinded as they will be responsible for distributing the nicotine-free cigarettes to participants. The research manager and research assistants based at Quitline will not be blinded to treatment allocation, as they are required to know which group each participant has been allocated to in order to conduct the scheduled follow-up calls. Page 3 of 8 Study intervention Participants will be randomised to a six week interven- tion of nicotine-free cigarettes (Quest 3) plus eight weeks of usual care (NRT plus behavioural support) ver- sus eight weeks of usual care (NRT plus behavioural support). Treatment group: Participants allocated to the treatment arm will be asked to stop smoking their normal cigarettes on their chosen Quit day and switch to the nicotine-free cigarettes (Quest 3) cour- iered to them, then at six weeks to stop smoking the nicotine-free cigarettes (Figure 1). Participants will be told that they should try all the techniques sug- gested by the Quitline advisors to help them get through the withdrawal period (i.e. drink water, deep breathe, do something else, delay), but if they still feel they must have a cigarette, they should only smoke the nicotine-free cigarettes provided. Partici- pants will also be supported by Quitline in the usual way for eight weeks, with counselling/advice and NRT in the form of a patch, gum and/or lozenge (Figure 1). The Quest 3 cigarettes used in this trial have recently been reformulated by Vector Tobacco Inc and no longer contain genetically modified tobacco. The “new” cigarettes still have a similar nicotine content (1.5 mg per cigarette) and yield (≤ 0.05 mg per cigarette) as the original Quest 3 cigar- ettes, but the tar content has been reduced from 10mg to 4mg per cigarette. In order to verify the nicotine Randomisation and Quit day 3 weeks 6 weeks 8 weeks Treatment group Nicotine-free cigarettes (≤ 0.05 mg nicotine) Control group NRT patch, gum and/or lozenge and usual support NRT patch, gum and/or lozenge and usual support NRT = Nicotine Replacement Therapy Figure 1 Schematic of treatment allocation during intervention period.