Does e-cigarette nicotine strength influence substitution for combustible cigarettes?

Using the Experimental Tobacco Marketplace, we examined the effects of e-liquid freebase nicotine strength (3-24 mg/mL) and e-liquid price (US$0.25-$1.00/mL) on behavioral economic substitution for cigarettes in dual cigarette/e-cigarette (n = 41) and exclusive cigarette (n = 40) users. Subjective effects of nicotine strength were also examined. All nicotine strengths served as a substitute for combustible cigarettes, with greater substitution observed in dual users. When analyzing e-liquid volume purchased (mL), nicotine strength did not influence substitution; however, when analyzing e-liquid nicotine purchased (mg), degree of substitution increased as a function of nicotine strength. Additionally, higher nicotine strengths reduced the volume and probability of e-liquid purchasing, increased cigarette demand, and produced less favorable subjective effects than lower strengths. Increasing e-liquid price reduced e-liquid purchasing and did not influence substitution. We conclude that nicotine strength does not robustly affect the degree to which e-liquid substitutes for cigarettes, as no effect was observed in analysis of the unit of purchase most relevant to real-world sales (volume). Instead, high-strength nicotine potentially reduces the appeal and probability of purchasing e-liquid. Future work should examine these effects in nicotine salt-based e-liquids compared with freebase nicotine (used here) and in choice contexts with concurrent access to different nicotine strengths.