Cognitive decline before and after mid-to-late-life smoking cessation: a longitudinal analysis of prospective cohort studies from 12 countries

Background
Whether short-term improvements in cognitive performance observed following smoking cessation are transient or if longer-term cognitive trajectories are also improved is unclear, particularly when adults are middle-aged or older at smoking cessation. We examined whether long-term cognitive trajectories improved following mid-to-late-life smoking cessation.
Methods
In this longitudinal study, we used data from three nationally representative cohort studies from 12 countries including 18 years of cognitive data (2002–20). Participants who quit smoking during follow-up were matched with an equal number of continuing smokers according to key demographic, socioeconomic, and cognitive criteria. We used piecewise linear mixed models to examine memory and fluency decline before and after smoking cessation and during a comparable time period in continuing smokers.
Findings
We included data from 9436 participants who smoked (4718 [50·0%] smokers who quit matched with 4718 [50·0%] continuing smokers, aged 40–89 years, with 4886 [51·8%] women and 4550 [48·2%] men). In the six years before smoking cessation, matched smokers who quit and continuing smokers had similar rates of memory and fluency decline (difference in memory decline [smokers who quit–continuing smokers] –0·03 SDs [95% CI –0·06 to 0·01], p=0·16; difference in fluency decline –0·01 [–0·04 to 0·03], p=0·76). In the six years following smoking cessation, smokers who quit had memory and fluency scores that declined more slowly than continuing smokers (difference in memory decline 0·05 SDs [0·00–0·10], p=0·036; difference in fluency decline 0·05 SDs [0·01–0·10], p=0·030). Coefficients for interaction with age at smoking cessation suggested results did not differ by age at smoking cessation (p>0·05 for all).
Interpretation
In middle-aged and older smokers with initially similar cognitive trajectories, smokers who quit subsequently had more favourable trajectories than continuing smokers regardless of age at cessation. As older adults are less likely than younger people to attempt smoking cessation, improvements in long-term cognitive trajectories might provide an additional motivation to quit.