OBJECTIVE: Nevirapine-based triple combination antiretroviral (ARV) therapy has been used as first line drug for HIV patients in Thailand. However, negative consequence from adverse events is of concern when compared to the Efavirenz-based ARV. The aim of this study is to compare the incidence of Nevirapine-based and Efavirenz-based therapy. METHODS: Nineteenth general hospitals were recruited in the Intensive Adverse Drug Reaction (ADR) Monitoring program. One or two pharmacists from each of these hospitals were trained to understand the concept of pharmacoepidemiology, pharmacologic measure, and how to complete adverse event reports and antiretroviral drug utilization data. Adverse event data of the hospital completely reporting utilization of ARV within six months period from January to June, 2005, were used for the study. The incidences of adverse events of Nevirapine-based and Efavirenz-based ARV therapy were calculated and compared. RESULTS: Six hospitals (31.6%) completed the drug utilization report. Of the 1,185 patients using Nevirapine-based ARV, 39 cases had adverse event. Of the 369 patients usingh Efavirenz-based therapy, nine cases had adverse event. The ADR incidence of Nevirapine and Efavirenz was 3.29 and 2.44 per 100 persons per six months. Sixteen cases (1.35%) had to quit Nevirapine and three cases (0.81%) had to quit Efavirenz. Six cases (0.51%) in Nevirapine-based ARV group had severe and life-threatening ADR resulting in hospitalization including Steven Johnson Syndrome, oedema, dyspneoa, and skin exfoliation. Within Efavirenz-based group, only one case (0.27%) had Fixed Eruption and was hospitalized. CONCLUSIONS: For overall ADR, Nevirapine-based ARV had the ADR incidence higher than Efavirenz-based ARV. The serious or life threatening ADR seems to be greater in Nevirapine group. Data from Intensive ADR monitoring program could be used for assessing the ADR incidence which could be incorporated in pharmacoeconomic calculation.