A randomized experiment to examine unintended consequences of dietary supplement use among daily smokers: taking supplements reduces self‐regulation of smoking

Aims  We examined whether smokers' use of dietary supplements (e.g. vitamin C, multi‐vitamins) induces illusory invulnerability that in turn disinhibits smoking. Such supplement use may be perceived as conferring health credentials. Design  A single‐factor (credentials: with or without) between‐subjects design was employed. Smokers were assigned randomly to take either a known placebo pill or a dietary supplement (in fact, the same placebo) in an ostensible health‐food test. Setting  Laboratory at Kaohsiung Medical University, Taiwan. Participants  Study 1 involved of a student sample consisting of 74 daily smokers, whereas study 2 involved a community sample consisting of 80 daily smokers. Measurements  In Study 1, participants reported their perceived invulnerability following the manipulation. In study 2, pre‐ and post‐test measures of invulnerability were administered, and attitudes towards dietary supplements were assessed prior to the manipulation. In both studies, the dependent measure was the number of cigarettes smoked during completion of an unrelated survey. Findings  Participants who believed that they were taking a dietary supplement smoked more cigarettes than did controls. Study 1 found support for the role of perceived invulnerability as a mechanism underlying this effect. Study 2 demonstrated the moderating effect played by attitudes towards dietary supplements: a more positive attitude towards supplements increased susceptibility to licensing effects. Conclusions  Dietary supplement use may create illusory invulnerability, reducing the self‐regulation of smoking. Reminding health‐conscious smokers that multi‐vitamins do not prevent cancer may help such smokers to control their smoking and encourage them to stop.