This letter is a reply to Wallace and MacKay [1]. They provided thoughtful comments on our paper published in the December 2011 issue of Addiction[2].

Drawing on the perspective provided in the literature on license and health regulations, we suggested that people may be less likely to scrutinise the health-related implications of their behaviours and to regulate these behaviours immediately after their identification of a healthy self has been reinforced by their engagement in behaviours perceived as healthy (e.g. taking dietary supplements). Our research addressed whether taking dietary supplements would elicit a feeling of license in smokers and lead them to increase their cigarette consumption [2]. Participants in two study conditions were smokers. Neither the prevalence of smokers taking dietary supplements nor the association between smoking habits and dietary supplementation was our focus. Our research question was quite different from that raised in the survey on the ‘Healthy Habits of Supplement Users and Non-Users’[3] that reported on a comparison between smokers who did and did not use supplements.

Bailey et al. [4] indicated that dietary supplement use was associated with the avoidance of smoking, but the usual methodological differences between correlational studies and laboratory experiments interfered with the ability to draw direct comparisons. Although large-scale surveys should be accorded greater credibility due to their representativeness and external validity, the covariation among self-report measures may be susceptible to inflation. Subtle co-variations may reach statistical significance because the error term used for statistical testing is profoundly reduced by a large sample size. Furthermore, the link between supplement use and avoidance of smoking in that survey was one of covariance and not causality. One possible causal relationship would link good health habits with a greater likelihood of taking supplements, but the opposite would not be true. For example, people with good health habits (Z) may take health-protective actions (e.g. supplements, X) and avoid engaging in unhealthy acts (e.g. smoking cigarettes, Y) as well. In this case, the co-variation between X and Y may be spurious because the relationship between Z and X and the relationship between Z and Y may have produced the co-variation between X and Y. Moreover, the findings of that survey could not be used to determine whether taking supplements would result in smoking cessation.

Our second study recruited a community sample to expand the generalisability of our findings. Even in the first study involving an undergraduate sample, the real purposes were disguised, and none of the participants reported any suspicions about the link between supplement manipulation and the dependent measure during the debriefing. A dispersion of information about our particular manipulation and dependent measures would likely happen among participants recruited from a university campus. However, candidate participants who may have possibly known about the proposed hypothesis were excluded during the screening procedure for our first study. We wonder why the authors of the letter [1] would judge this study as not employing a blind design.

In our article, we never suggested that smokers should not take dietary supplements. Mediation analyses indicate that perceived invulnerability mediated the licensing effect of supplement use on the number of cigarettes smoked. Hence, if smokers will not feel an illusory sense of health protection by taking supplements, they may be less susceptible to the licensing effect. In fact, giving license can be avoided when smokers remind themselves of why they engaged in that ‘health-protective act’ in the first place. If smokers can remember that their primary goal is health, the licensing effect may disappear when smokers make subsequent health-related choices. If smokers want to take dietary supplements, they should. They must just remember to remind themselves of their desire to make healthy choices when they take the supplement rather than focus on how the supplement presumably leads to invulnerability to health hazards. In general, different scientific disciplines may approach a similar issue in different ways, leading to disparate findings. We think that scientific discourse between psychologists and pubic health researchers may contribute to a more comprehensive understanding of dietary supplementation. We thank the authors of the letter [1] for their invaluable suggestions for future studies.