Chlorprothixene-D6 is a stable isotopically labeled internal standard relevant to chlorprothixene, a thioxanthene antipsychotic assessed in clinical toxicology and forensic casework. It supports accurate quantitation of chlorprothixene exposure in therapeutic drug monitoring, overdose investigations, and impairment evaluations. As a CRM, it supports LC-MS/MS or GC-MS methods while addressing matrix effects, calibration, and identification challenges.