A comparison of a short half‐life marker (low‐dose isoniazid), a long half‐life pharmacological indicator (low‐dose phenobarbitone) and measurements of a controlled release ‘therapeutic drug’ (metoprolol, Metoros) in reflecting incomplete compliance by volunteers.

Although, long half-life compounds appear to be more appropriate pharmacological indicators of compliance with treatment, short half-life markers or measurements of short half-life therapeutic drugs are frequently used. 2. We have compared the usefulness of low-dose phenobarbitone (a long half-life indicator), low dose isoniazid (a short half-life marker) and controlled release metoprolol (Metros) (a controlled release formulation of a short half-life 'therapeutic' drug) in seven volunteers with simulated partial (two thirds) compliance. 3. Detection of isoniazid metabolites in urine had an 83% sensitivity and 94% specificity for detecting ingestion within the previous 24 h and 100% sensitivity and 82% specificity for detecting ingestion within the past 6 h but gave no indication of the longer term pattern of compliance. 4. At 28 days (a time when steady-state would be obtained for all three drugs) phenobarbitone plasma levels were 70% (66-76%)--median and interquartile range--of the expected steady-state level if compliance had been complete. Corresponding figures for metoprolol were 82% (37-100%). 5. Measurement of phenobarbitone was much superior to isoniazid or metoprolol measurements in reflecting partial compliance over the previous 1 to 4 weeks.