Assessment of Methadone Dose Taken

Inclusion Criteria: * Age 18-70 * A prescription for methadone for chronic pain at a dose of 10mg or more for at least one week. * Taking methadone as prescribed during the last 4 days before consent to participate in the study. Exclusion Criteria: * Age \<18 or \>70 * A condition preventing or complicating ISF collection * dermatological (skin) condition * immunodeficiency * recent blood donation * anemia * end stage renal disease * liver cirrhosis * cancer * congestive heart failure * bleeding diathesis * tuberculosis (TB) * Any active severe depression * suicidal ideation * mania symptoms * Pregnancy * Intending to become pregnant during the course of the study * Enrolled in a substance use disorder treatment program * Under a conservatorship.