Re: Change in mental health after smoking cessation: systematic review and meta-analysis
We reported that there was a moderate to strong association between quitting smoking and improvements in mental health compared with continuing to smoke. We favoured the explanation that quitting improved mental health. Dr Searle and Professor Sanderson and colleagues wonder whether the association between quitting smoking and improvements in mental health could be more plausibly caused by improved mental health leading to a greater ability to quit or a common factor causing both improved mental health and increasing the likelihood of cessation.
There is very limited scope for reverse causation. It seems entirely plausible that people try to quit smoking when their mood improves and this could explain the association in some of the population cohorts we examined. However, more than half the studies were smoking cessation trials where all enrollees had mood assessed at baseline and then all attempted to quit more or less immediately so improved mood cannot have caused the decision to quit smoking here. The effect size in these studies was the same as in the population cohorts, suggesting reverse causation was an unlikely causal factor even in the population cohorts.
While improved mood cannot explain the decision to quit, could some third factor explain both the success of attempts to stop and the improvement in mood seen? None of our correspondents were able to hypothesise what such factor might be. It must have occurred after the quit attempt started, because if it was operating prior to cessation it would have been reflected in the pre-cessation mood measure and not reflect in the change score we used for the analysis. Furthermore, our studies assessed mood anywhere between 6 weeks and 9 years after cessation with no evidence of change in effect size over time. We know of no evidence that, for example, positive life events enable people to sustain a quit attempt. Furthermore such events must be common enough to be occurring in a sufficient number of people who quit smoking to influence the mean mood ratings at six weeks after, but not prior to, cessation starting and that such events are still influencing mood to the same extent nine years later. We cannot rule out such an explanation, but it feels much less plausible than the explanation that stopping smoking improves mood.
Sanderson and colleagues propose that changes in behaviours other than smoking may influence mood in those who stop smoking and not in those who continue. We agree this is plausible. It could be that people who stop smoking take-up exercise while continuing smokers do not and exercise may improve mood. This, however, is an example of the causal effect of cessation on mood improvement, albeit by an indirect route.
Whatever the true explanation for the cause, the implications for clinicians and patients remain unchanged. People who either do not stop smoking or try and fail have on average the same mood at follow-up as they did at baseline. People who stop smoking and succeed have a more positive mood state. The reason why mood improves matters much less to people than that it has. Whatever the reason, we can say that people who stop smoking have improved mental health.
Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf and declare: GT has received grants and personal fees from a National Coordinating Centre for Research Capacity Development scholarship, during the conduct of the study; and personal fees from UK Centre for Tobacco and Alcohol Studies, outside the submitted work. AMcN has received grants from UK Centre for Tobacco and Alcohol Studies, outside the submitted work. AF has received grants from National Prevention Research Initiative, during the conduct of the study; and sat on the professional development group for NICE guidance on stopping smoking in secondary care. NL-H has received personal fees from manufacturers of smoking cessation aids, outside the submitted work; and manages an NIHR HTA funded trial of nicotine patch preloading. The nicotine patches for the trial are provided free of charge by GlaxoSmithKline (GSK). GSK have no other involvement in the trial. PA has received personal fees from Pfizer, grants and personal fees from McNeil, outside the submitted work.