Riboflavin has been incorporated into isoniazid tablets designed for antituberculous therapy in a proportion of 50:8 by weight. Measurement of the concentrations of riboflavin in the urine of normal patients, of patients ingesting supplementary vitamin preparations, and in the urine of patients taking isoniazid with riboflavin added in this proportion has established that riboflavin may be used as a means of detecting those patients who have ingested isoniazid less than 24 hrs. prior to the test. In the experiments described herein, the concentration of riboflavin in normal urine was equal to or less than 2.4 [mu]g per ml in essentially all instances. The concentration in patients receiving isoniazid in dosages equivalent to 5 to 10 mg per kg of body weight, with riboflavin added, i.e., in dosages equivalent to 0.8 to 1.6 mg per kg, exceeded 2.5 [mu]g per ml. The results of these studies indicate that riboflavin-tagged isoniazid is a practicable method for separating the patients who regularly follow self-medication programs from those who do not.