Measuring compliance in methadone maintenance patients: use of a pharmacologic indicator to "estimate" methadone plasma levels.

A quantitative indicator of compliance is not available for methadone--the drug of choice for the treatment of opioid addiction. We successfully used low-dose phenobarbital (a valid pharmacologic indicator) to measure compliance by incorporating the drug into the methadone medication of patients attending an addiction unit. Plasma phenobarbital and methadone concentrations were measured in 20 (11 clinic-based and 9 community-based) patients receiving long-term treatment with the phenobarbital level-to-dose ratio, together with interviews, to validate methadone measurements and to monitor compliance. Patients attending the unit on a daily basis and who consumed their medication in the clinic were substantially more compliant than community-based patients. Laboratory measurements of phenobarbital and methadone helped to identify the use of illicit methadone, as well as incorrect self-administration, such as the consumption of several days' dosage at one time.