1. van der Merwe PJ, Muller FR, Muller FO: Caffeine in sport. Urinary excretion of caffeine in healthy volunteers after intake of common caffeine-containing beverages. S Afr Med J. 1988 Aug 20;74(4):163-4. [PubMed:3406872 ]
  2. Mumford GK, Benowitz NL, Evans SM, Kaminski BJ, Preston KL, Sannerud CA, Silverman K, Griffiths RR: Absorption rate of methylxanthines following capsules, cola and chocolate. Eur J Clin Pharmacol. 1996;51(3-4):319-25. doi: 10.1007/s002280050205. [PubMed:9010706 ]