Used as a stable isotope-labeled internal standard in mass spectrometry-based bioanalytical assays for the quantification of chlorpromazine and its metabolites in biological samples. Its primary application is in pharmacokinetic and metabolic studies, where it helps ensure accurate measurement by compensating for matrix effects and instrument variability. Due to the deuterium substitution, it closely mimics the behavior of the parent compound while being distinguishable by mass detection, making it ideal for use in research involving drug metabolism and forensic toxicology.