methadone drug pathway 2015-09-22T14:15:14Z PW:0001939 The pharmacokinetics and pharmacodynamics pathway of methadone, an acyclic analog of morphine and heroin acting on the same opioid receptors and used in the management of severe, chronic pain. Genetic variations can result in changes in drug availability and can cause differences in the response of the organism to the drug. pathway vpetri opioid drug pathway