A longitudinal study of cotinine in long-term daily users of e-cigarettes

• We assessed twice, eight months apart, levels of cotinine in experienced vapers.
• Over time, these vapers decreased the concentration of nicotine in their e-liquids.
• They compensated by using more liquid, to maintain their cotinine levels constant.

BackgroundIt is not clear whether, in established vapers, cotinine levels remain stable or change over time.MethodsWe enrolled 98 exclusive users of e-cigarettes on websites and forums dedicated to smoking cessation and to e-cigarettes. We collected saliva vials by mail in 2013–2014 (baseline), and collected a second saliva vial eight months later (follow-up) in the same participants. Participants had not used any tobacco or nicotine medications in the previous five days. Cotinine in saliva was analyzed with liquid chromatography–mass spectrometry. Use of e-cigarettes, tobacco and nicotine medications was self-reported.ResultsAll participants were former smokers, and 99% were using e-cigarettes daily. They had already been using e-cigarettes for nine months on average at baseline. The median cotinine level was 252 ng/mL at baseline (quartiles: 124–421 ng/mL) and 307 ng/mL at follow-up (114–466 ng/mL, W = 0.9, p = 0.4 for change over time). The median concentration of nicotine in refill liquids was 11 mg/mL at baseline (quartiles: 6–15 mg/mL) and 6 mg/mL at follow-up (5–12 mg/mL) (Wilcoxon signed rank test: W = 5.2, p < 0.001 for change over time). The median volume of e-liquid used per month was 80 mL at baseline (quartiles: 50–130 mL) and 100 mL at follow-up (60–157 mL, W = 3.3, p = 0.001 for change over time).ConclusionIn experienced e-cigarette users enrolled online, cotinine levels were similar to levels usually observed in cigarette smokers. Over time, these users decreased the concentration of nicotine in their e-liquids, but increased their consumption of e-liquid in order to maintain their cotinine levels constant.