A METHOD OF PREVENTING FORMATION OF MULTI-DRUG RESISTANCE (MDR) OR OF TREATING A PATIENT THAT HAS DEVELOPED OR IS SUBJECT TO THE DEVELOPMENT OF MDR COMPRISES ADMINISTERING A REGIMEN COMPRISING ONE OR MORE ACTIVE DRUGS TOGETHER WITH ONE OR MORE MDR INHIBITORS SUCH THAT THE PHARMACOKINETIC (PK) PROFILE OF THE MDR INHIBITOR(S) IS/ARE MATCHED TO THE PK PROFILE OF THE ACTIVE DRUG(S).