Causal Factors of Increased Smoking in ADHD: A Systematic Review

Background: ADHD is a highly prevalent disorder and poses a risk for a variety of mental disorders and functional impairments into adulthood. One of the most striking comorbidities of ADHD is nicotine dependence. Youth diagnosed with ADHD are 2-3 times more likely to smoke than their peers without ADHD, initiate smoking earlier in life and progress more quickly and more frequently to regular use and dependence. Possible explanations for these increased risks are: (a) self-medication of ADHD symptoms with the stimulant nicotine; (b) ADHD symptoms like inattention and hyperactivity/impulsivity predispose for smoking initiation and impede smoking cessation; (c) peer pressure; and/or (d) common genetic or environmental determinants for ADHD and smoking. Objective: Identify the most probable causes of the high prevalence of smoking and nicotine dependence in subjects with ADHD. Methods: A systematic literature review was performed and the causality of the observed relations was ranked using the Bradford Hill criteria. Findings: ADHD medication reduces early smoking initiation and alleviates smoking withdrawal. Nicotine patches, bupropion and (probably) varenicline ameliorate ADHD symptoms. Imitation of and interaction with peers and genetic and environmental determinants may contribute to the comorbidity, but seem to contribute less than self-medication. Conclusion: Smoking is probably best explained by a combination of imitation, peer pressure and typical traits of ADHD. In contrast, the positive relation between ADHD and nicotine dependence is currently best explained by the self-medication hypothesis. This hypothesis has a clear pharmacological rationale and is supported by ample evidence, but awaits confirmation from longitudinal naturalistic studies.