Kashuba AD, Bertino JS, Kearns GL, Leeder JS, James AW, Gotschall R, Nafziger AN. Quantitation of three-month intraindividual variability and influence of sex and menstrual cycle phase on CYP1A2, N-acetyltransferase-2, and xanthine oxidase activity determined with caffeine phenotyping. Clin Pharmacol Ther. 1998 May; 63(5):540-51.