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Nicotine Tob Res. 2019 Mar 14. pii: ntz042. doi: 10.1093/ntr/ntz042. [Epub ahead of print]

An Update on Hardening: A Qualitative Review.

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Center for Behavior and Health, Departments of Psychiatry and Psychological Science, University of Vermont, Burlington, VT.



This review examines the evidence for the hardening hypothesis; i.e., the prevalence of a) becoming a former smoker is decreasing over time due to b) decreased quit attempts, or c) decreased success on a given quit attempt.


PubMed, EMBASE, Psych Info, trial registries, and other databases were searched for population-based surveys that reported whether one of the above three outcomes decreased over a 5+ year time period.


None of the 26 studies found that conversion from current to former smoking, number of quit attempts, or success on a given quit attempt decreased over time and several found these increased over time. These results appeared to be similar across survey dates, duration of time examined, number of data points, data source, outcome definitions and nationality.


These results convincingly indicate hardening is not occurring in the general population of smokers. On the other hand, the prevalence of smoking is declining less among older and women smokers, and smokers with low education, low income, psychological problems, alcohol/drug abuse, medical problems, and greater nicotine dependence, than among those without these characteristics, presumably due to less quitting. Why this has not lead to decreased success in stopping smoking in the general population is unclear.


Some have argued that a greater emphasis on harm reduction and more intensive or dependence-based treatments are needed because remaining smokers are those who are less likely to stop with current methods. This review finds no or little evidence for this assumption. Psychosocial factors, such as low education and psychiatric problems, predict less ability to quit and appear to becoming more prevalent among smokers. Why this is not leading to decreased quitting in the general population is an anomaly that may be worth trying to understand.


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