Drug consumption is sometimes used as a secondary judgment criterion for clinical trials. Many measurement methods are available to quantify drug consumption. Several methods were applied in a rheumatic disease trial involving 121 patients with chronic low back pain who lived around Saint-Nectaire, France, and who participated in the trial from April to November 1993 to determine an easily used and practical measurement method to detect a significant drug consumption change over time. Analgesic and nonsteroidal antiinflammatory drugs (NSAIDs) were classified according to the anatomical therapeutic chemical classification. Consumption was quantified on a weekly basis in number of tablets (method 1), unit of defined daily dose (method 2), milligrams of active principle (method 3), and NSAID equivalence score (method 4). These methods were applied in a randomized clinical trial of spa therapy on sufferers of chronic low back pain. An analysis of variance with repeated measures showed a significant difference in drug consumption between treatment and control groups detected by all methods, except for the NSAID consumption measured with method 3. The comparison of each method by the relative efficiency index indicated that method 1 had a greater sensitivity for detecting changes of drug consumption. Tablet count appears to be a more sensitive and more practical method for detecting a drug consumption change in clinical trials.