Self-Administration of Nicotine Outside of Tobacco by Smokers: A Metaanalysis of Laboratory Studies.

INTRODUCTION/OBJECTIVES: Nicotine is widely considered the primary psychoactive constituent responsible for the persistence of tobacco smoking. However, it remains unclear to what extent nicotine, when isolated from tobacco-related sensory and contextual factors, functions as a reinforcer in smokers. Laboratory self-administration paradigms are regarded as the gold standard for assessing the reinforcing effects of psychoactive substances. The present study aimed to quantitatively assess whether smokers preferentially self-administer nicotine when delivered outside of tobacco, relative to a matched placebo.

METHODS: We conducted a systematic review and meta-analysis of laboratory studies in which adult smokers, following overnight smoking abstinence, were given repeated opportunities to choose between self-administration of nicotine (delivered via gum, nasal spray, electronic cigarettes, or intravenously) and a pharmacologically inert placebo. Event rates representing the proportion of nicotine choices were pooled using a random-effects model.

RESULTS: Eighteen published articles comprising 35 experimental conditions met the inclusion criteria. Across all studies and modes of delivery, the pooled proportion of nicotine choices was 0.49 (95% CI: 0.43-0.55), indicating no overall preference for nicotine over placebo. Similar results were obtained in a secondary analysis restricted to studies using nicotine nasal spray, a delivery method selected for its relatively rapid pharmacokinetic profile. Intravenous nicotine studies yielded a numerically higher mean choice rate, but this was not statistically different from chance.

DISCUSSION: These findings are consistent with contemporary models of tobacco use that emphasize the importance of sensory cues, expectations, and contextual factors in sustaining smoking behavior. Recent experimental and regulatory research on nicotine delivery systems and reduced-nicotine cigarettes likewise suggests that the reinforcing properties of nicotine are strongly modulated by delivery kinetics, product design, and non-nicotine constituents.

CONCLUSION: Under controlled laboratory conditions, smokers do not, on average, preferentially self-administer nicotine when it is isolated from tobacco. These findings suggest that the reinforcing effects commonly attributed to nicotine may depend substantially on sensory, contextual, and nonpharmacological factors associated with tobacco use.