Our data support a functional importance of the ABCB1 mutations for plasma concentrations and central nervous actions of the opioid loperamide.

Whereas morphine clearly produced miosis and respiratory depression, pretreatment with quinidine as an inhibitor of P-glycoprotein did not result in an enhancement of central nervous opioid effects in healthy volunteers.

The active morphine metabolite is subject to transmembrane transport by transporters inhibited by probenecid in humans.