Objectives The association between smoking reduction and mental health is of particular interest given that many smokers report that smoking offers mental health benefits. We aimed to assess the association between smoking reduction and change in mental health using two different analytical approaches to determine if there was any evidence of an association. There were no prior hypotheses.

Design A secondary analysis of prospective individual level patient data from 5 merged placebo-controlled randomised trials of nicotine replacement therapy for smoking reduction.

Participants All participants were adult smokers, selected because they wanted to reduce but not stop smoking, and had smoked for at least 3 years. Participants were excluded if they were pregnant, breastfeeding, under psychiatric care, deemed to be unfit by a general practitioner, or part of a cessation programme. 2066 participants were enrolled in the trials, 177 participants were biologically validated as prolonged reducers, and 509 as continuing smokers at both 6-week and 18-week follow-ups.

Primary outcome Change in mental health from baseline to an 18-week follow-up was measured using the emotional well-being subscale on the Short Form Health Survey-36.

Results After adjustment for confounding variables, the differences for reducers compared with continuing smokers were: regression modelling −0.6 (95% CI −4.4 to 3.2) and propensity score matching 1.1 (95% CI −2.0 to 4.1).

Conclusions Smoking reduction, sustained for at least 12 weeks, was not associated with change in mental health, suggesting that reducing smoking was no better or worse for mental health than continuing smoking. Clinicians offering smoking reduction as a route to quit can be confident that, on average, smoking reduction is not associated with negative change in mental health.