Opioids are among the most effective drugs in managing moderate to severe pain. Herein, we describe a morphine-metamizole adduct with a phenazone moiety added at position C2 of the morphinan backbone. This adduct, coined metamorphine, was first detected as a byproduct of the morphine-metamizole drug interaction in patient-controlled analgesia (PCA) pumps used for management of severe pain. In this study, metamorphine was successfully synthesized using the Mannich condensation. Qualitative high-performance liquid chromatography with ultraviolet detection (HPLC-UV) analysis was employed to confirm the identity of metamorphine, and the yield was then purified using preparative HPLC. Subsequent studies were undertaken to investigate the pharmacological properties of the newly synthesized compound. A radioligand-based binding assay demonstrated that metamorphine binds strongly to the μ-opioid receptor (