Many physicians prescribe more than one antiulcerative agent (AUA) simultaneously to the same patient, although there is little evidence to support this practice. The purposes of this study were to (a) determine patient factors associated with the concurrent use of these agents and (b) estimate the excess costs generated by the prescription of multiple rather than a single agent. We conducted a case-control study of concurrent AUA users among New Jersey Medicaid enrollees age 65 years and older. To evaluate the excess cost generated by the ongoing prescription of an additional AUA, we measured the additional drug expenditures associated with each regimen of concurrent use. Nearly 1 in 15 AUA users (6.6%) met our conservative definition of concurrent AUA use. In a multiple logistic regression model, previous gastrointestinal procedure, use of a non-steroidal anti-inflammatory drugs, nursing home residency, and recent hospitalization for more than 20 days were all predictors of concurrent use of more than one AUA. No association was found with age, sex, or number of pharmacies used. The upper bound estimate of the cost generated by the concurrent prescription of a second AUA was $210 (range: $2-$942) over the 180-day study period, with a lower bound of $151 (range: $1-$449). Annually, such excess cost would range from $301 to $420 per patient. This would account for between $457 million and $637 million per year for the nation's elderly if these patterns are generalizable. Despite the lack of evidence of therapeutic benefit from multiple concurrent AUA use in most patients, this practice is fairly common. Besides introducing the risk of additional costs and side effects in the absence of additional efficacy, the costs of such duplicative prescribing are substantial.