Intra- and inter-individual variability in urinary nicotine excretion and plasma cotinine in adult cigarette smokers

► Data were gathered from 13 randomized controlled clinical studies of adult smokers. ► A statistical model was used to examine inter- and intra-individual variability in biomarker. ► The average variability of nicotine equivalents and plasma cotinine were estimated. ► The information is relevant to exposure study design and tobacco product regulation.