Medication use is an important consideration in chronic-pain rehabilitation programs (CPRPs). However, it is difficult to quantify this aspect of chronic-pain treatment, because patients often take multiple medications that can differ by pharmacological class as well as dosage level. The Medication Quantification Scale (MQS) provides a method for quantifying medication use in patients with chronic, nonmalignant pain. Scores are calculated for each medication based on weights assigned by medication class and dosage level, and these scores are summed to provide a quantitative index of total medication usage suitable for statistical analysis. The method for calculating MQS scores is illustrated, and research data on MQS reliability and validity are presented. Interrater reliability was rho = 0.985 (p less than 0.0001) for 30 MQS scores calculated by two clinicians. MQS scores for 88 patients correlated well with the clinical judgment of 12 health care professionals (mean rho = 0.755, p less than 0.0001). The MQS scores for 60 chronic-pain patients (30 treated in a CPRP and 30 untreated) were obtained at two time points: evaluation and 1-year follow-up. A two (groups) by two (time points) analysis of variance yielded a significant group-by-time interaction (F = 8.82, p less than 0.0043). Treated patients decreased their medication intake significantly (p less than 0.0001), whereas untreated patients did not (p greater than 0.57). The MQS offers a reliable and valid method for quantifying medication usage in chronic-pain patients.