Herb–drug interaction has become a serious problem since herbal medicine is extensively used in themodern world. This study investigates effects of Andrographis paniculata extract (APE) and its major component,andrographolide (AG), on the pharmacokinetics of theophylline, a typical substrate of cytochromeP450 1A2 enzyme, in rats. After APE or AG pretreatment for 3 days,