Pharmacoepidemiologic study of potential drug interactions in outpatients of a university hospital in Thailand


Benjamas Janchawee PhD, Department of Pharmacology, Faculty of Science, Prince of Songkla University, Hat Yai 90112, Thailand. Tel./fax: +66 74 446678; e-mail:


Background:  Drug–drug interaction is a potential cause of adverse drug reactions. The incidence of such drug interactions in university hospitals in Thailand is unknown.

Purpose:  To estimate the rate of potential drug–drug interactions in outpatients of a typical Thai university hospital, and to identify risk factors for such interactions in Thai patients.

Methods:  One-year outpatients’ prescription data were retrieved from the hospital computer records. Potential drug interactions were identified using the existing drug-interaction database system. Potential interactions within a specific prescription and involving drugs prescribed 1-, 3- and 7-day earlier were searched for. Possible associations between occurrence of an interaction and a patient's age and gender and the number of items on the prescription were explored.

Results:  The overall rate of potential drug interactions was 27·9% with a maximal value of 57·8% at the Department of Psychiatry. The rate of the most potentially significant interactions was 2·6%, being the highest in the Department of Medicine (6·0%), with isoniazid vs. rifampin as the most common interacting combination. The rate increased with the patient's age and prescription size (P = 0·000). The odd's ratio of having at least one potential drug interaction was 1·8 (64·2%) when age increased by 20 years (P = 0·000) and 2·8 (165·7%) when another drug was added (P = 0·000). The rate of potential drug interactions was the same for both genders. The rate of potential drug interactions detected across prescriptions was higher than within prescriptions and was dependent on the time interval between prescriptions.

Conclusions:  Potential drug interactions were common in our sample of patients. The rate of such interactions increased with the number of drugs prescribed and the patient's age.