Used as an internal standard in mass spectrometry and chromatographic analysis for the accurate quantification of diltiazem in biological samples such as plasma, serum, and urine. Its deuterated structure allows for clear differentiation from non-labeled diltiazem during analytical testing, improving measurement precision in pharmacokinetic and metabolic studies. Commonly applied in clinical research, therapeutic drug monitoring, and forensic toxicology to assess drug levels and ensure analytical reliability.