Feasibility of biochemical verification in a web-based smoking cessation study

•93/600 participants abstinent at 3-mo mailed saliva kits and 71% returned samples.•36.4% had cotinine levels above 15ng/mL and had discordant results.•Biochemical verification yielded low number of usable sample and was cost-intensive.•Findings reiterate questionable utility of biochemical verification in this setting.