Green tea catechins were recently reported to inhibit drug transporters such as organic anion-transporting polypeptides (OATPs) and metabolic enzymes, affecting the bioavailability of many drugs. This study aimed to evaluate the clinical significance of the effects of different doses of green tea extract on the pharmacokinetic parameters of atorvastatin and to rationalize the associated interaction mechanism.A randomized, double-blind, three-phase crossover study involving 12 healthy volunteers was performed. Participants received a single dose of atorvastatin 40 mg alone (control group), atorvastatin 40 mg plus a capsule containing 300 mg of dry green tea extract, or atorvastatin 40 mg plus a capsule containing 600 mg of dry green tea extract. Plasma samples taken from the volunteers were analyzed for atorvastatin using liquid chromatography-tandom mass spectrometry (LC/MS/MS).Compared to atorvastatin alone, the administration of 300 mg or 600 mg of the green tea extract along with atorvastatin decreased the peak plasma concentration (C) of atorvastatin by 25% and 24%, respectively (P < 0.05), and the area under the plasma concentration-time curve (AUC) of atorvastatin by 24% and 22%, respectively (P < 0.05). Additionally, administration of 300 mg or 600 mg of the green tea extract increased the apparent oral clearance (CL/F) of atorvastatin by 31% and 29%, respectively. The time to C (T) and the elimination half-life (t) of atorvastatin did not differ among the three phases. The effects of 600 mg of the green tea extract on the pharmacokinetic parameters of atorvastatin were not significantly different from the effects of 300 mg of the green tea extract.Green tea extract decreases the absorption but not the elimination of atorvastatin, possibly by inhibiting OATP, albeit not in a dose-dependent manner. Coadministration of green tea extract with atorvastatin may necessitate the monitoring of the plasma concentration of atorvastatin in clinical practice.