The present invention provides a highly accurate method for monitoring patient or subject compliance with a medication prescription by collecting a sample of the exhaled air. The method of the present invention is easy and little burdens the patient or subject. The method comprises the steps of: (i) prescribing a patient or subject ingestion of at least one biologically active agent and at least one carbon isotope-labeled lipid, (ii) obtaining a sample of the patient's or subject's exhaled air, (iii) measuring the ratio of the carbon isotope-labeled CO.sub.2 relative to .sup.12CO.sub.2 in the sample; and (iv) confirming the ingestion of the prescribed biologically active agent based on the ratio of the carbon isotope-labeled CO.sub.2 relative to .sup.12CO.sub.2.