Methadone: Historical success, modern challenges and lessons for opioid drug discovery.

Methadone is a long-standing synthetic opioid that remains clinically important for pain management and opioid use disorder, yet it exemplifies the dual nature of opioid drug discovery. Despite its therapeutic success, methadone exhibits complex pharmacokinetics, polypharmacology and safety liabilities that have limited its broader clinical application. This review examines methadone as a case study to illustrate how chemical structure, stereochemistry, receptor pharmacology and pharmacokinetic properties collectively shape therapeutic outcomes and risk. We summarize key advances in methadone synthesis, structure-activity relationships, pharmacology and clinical limitations and discuss how lessons learned from methadone continue to inform modern opioid drug discovery. Particular emphasis is placed on balancing efficacy, duration of action and safety in the design of next-generation opioid therapeutics.