Clinical drug interactions in outpatients of a university hospital in Thailand

Background: A clinical event is likely to occur in patients receiving a pair of drugs, that have the potential to cause an interaction. The occurrence of a clinical drug-drug interaction in outpatients of university hospitals in Thailand is unknown. Purpose: To investigate the occurrence of a clinical event associated with drug-drug interactions in outpatients at a Thai university hospital. Methods: A case-control study was established. The case was a sample group, randomly selected from a 1-year sample of outpatient prescriptions containing 'significance-1' potential drug-drug interactions, whereas the control was from the same year but with no potential drug interactions. Medical records of the cases and the controls were reviewed for an adverse event (AE) using a newly developed review form. The odds ratio of occurrence of the AE between the cases and the controls was determined. The AE was assessed for its possibility of being caused from a drug-drug interaction. Results: The most common specific AE in both the cases and the controls was cough. An unplanned revisit to outpatient department or emergency room was found to be the most common general AE. The odds ratio of the occurrence of an AE in the cases, compared with the controls, was 1.495 (95% Cl: 0.917-2.438). The possibility that the AEs resulted from drug interactions in the case group was nine 'probable' patients and 15 'possible' patients, whereas that in the control group was eight 'possible' patients. The most common interacting drug pair was isoniazid-rifampin with an increase in serum hepatic enzymes as the corresponding AE. Conclusions: Despite outpatients receiving drug pairs with a high potential for adverse interactions, the rate of occurrence of clinical drug interaction events was low.