Butonitazene-13C6 is a stable isotopically labeled internal standard relevant to butonitazene, a potent synthetic opioid encountered in forensic toxicology and drug seizure investigations. It supports accurate quantitation for exposure assessment, intoxication casework, and confirmation of butonitazene-related findings. As a CRM, it supports LC-MS/MS methods while addressing matrix effects, calibration, and identification challenges.