To examine possible blood-brain barrier (BBB) transport interactions between oxycodone and adjuvant analgesics, we firstly screened various candidates in vitro using [ 3 H]pyrilamine, a substrate of the oxycodone transporter, as a probe drug. The uptake of [ 3 H]pyrilamine by conditionally immortalized rat brain capillary endothelial cells (TR-BBB13) was inhibited by antidepressants (amitriptyline, imipramine, clomipramine, amoxapine, and fluvoxamine), antiarrhythmics (mexiletine, lidocaine, and flecainide), and ketamine. On the other hand, antiepileptics (carbamazepine, phenytoin, and clonazepam) and corticosteroids (dexamethasone and prednisolone) did not inhibit [ 3 H]pyrilamine uptake, with the exception of sodium valproate. The uptake of oxycodone was significantly inhibited in a concentrationdependent manner by amitriptyline, fluvoxamine and mexiletine with K i values of 13, 65, and 44 mM, respectively. These K i values are 5-300 times greater than the human therapeutic plasma concentrations. Finally, we evaluated in vivo interaction between oxycodone and amitriptyline in mice. Antinociceptive effects of oxycodone were increased by coadministration of amitriptyline. The oxycodone concentrations in plasma and brain were not changed by coadministration of amitriptyline. Overall, the results suggest that several adjuvant analgesics may interact with the BBB transport of oxycodone at relatively high concentrations. However, it is unlikely that there would be any significant interaction at therapeutically or pharmacologically relevant concentrations.