Theophylline (TP) is used to treat chronic obstructive pulmonary diseases, but its narrow therapeutic range limits its use. Changes in cytochrome (CYP)1A2-mediated metabolism can lead to unwanted TP concentration. Caffeine (CA) is frequently used with drugs and is a well-known CYP1A2 substrate, metabolized to paraxanthine (PA), theobromine (TB), and TP. Since CA and TP share overlapping metabolic pathway through CYP1A2, this study investigated how CA and TP pharmacokinetically interact with each other after oral administration of CA, TP, or CA plus TP (CA+TP) in mice. In the CA+TP group, co-administered CA resulted in a 1.61-fold increase in AUC with a 0.63-fold decrease in CL/F of TP compared to the TP group. This may be due to the inhibition of CYP1A2-mediated TP metabolism by CA. Co-administered TP also resulted in a 1.57-fold increase in AUC and a 0.64-fold decrease in CL/F of CA compared to the CA group. This could be due to TP activity to inhibit metabolic pathways, including CYP1A2, involved in CA metabolism. Moreover, the ratios of AUC PA /AUC CA , an indicator of CYP1A2 inhibition, and AUC TB /AUC CA were decreased in the CA+TP group compared to the CA group. These results suggest that the interaction between CA and TP is due to their overlapping CYP1A2-mediated metabolism being inhibited by their co-administration.