Background: Although antacids are popular drugs with a long history of use, their true utilization patterns—including over-the-counter use—have rarely been documented. Because all antacids are reimbursed under the National Health Insurance program in Taiwan, it is possible to access and analyze nationwide data for these drugs. Objectives: The purposes of this study were to estimate the scale of antacid prescribing in Taiwan using the national insurance claims for outpatient services and to analyze coprescribing patterns of antacids using modern data-mining techniques.<B>Methods:</B> The National Health Insurance Research Database in Taiwan supplied the visit-based sampling data sets, which had a sampling ratio of 0.2% for all claims for outpatient medical services in the year 2000. In addition to the plain statistics (ie, data from simple calculations) for antacid prescriptions, we also analyzed relationships between prescriptions for antacids and nonantacid drugs. A data-mining technique-association rule mining-was applied to identify the drugs prescribed in combination with antacids. Results: Among a total of 409,049 eligible prescriptions for 1,704,595 drug items to be administered orally, antacids were present in 213,494 prescriptions (52.2%). Antacid users were generally older than nonusers (mean [SD] age, 39.9 [23.4] years vs 32.4 [25.7] years). In all, 88.8% of antacid items (189,531/213,494) were prescribed without claims diagnoses of gastrointestinal disorders. Using association rule mining with a 1.0% minimum support factor, there were 36 strong association rules between prescriptions for antacids and other drug subgroups at the third level of Anatomical Therapeutic Chemical classification. Nonsteroidal anti-inflammatory drugs and drugs for treating upper respiratory infections played dominant roles in the associations with antacid prescriptions; vitamin B complex and antivertigo preparations were also strongly associated with antacids. Conclusions: Antacid coprescriptions were common in Taiwan in the year 2000. Further study should investigate whether antacid prescribing patterns are influenced by Taiwanese perceptions that Western drugs injure the stomach.