Amitriptyline-d3 Hydrochloride is primarily used in research and analytical laboratories as an internal standard for the quantification of amitriptyline and its metabolites in biological samples. Its deuterated form ensures accurate and reliable results in mass spectrometry and other analytical techniques, minimizing interference from non-deuterated compounds. This chemical is particularly valuable in pharmacokinetic studies, therapeutic drug monitoring, and forensic toxicology to assess drug levels in patients or investigate cases of overdose. Its application aids in understanding drug metabolism, ensuring proper dosing, and supporting clinical or legal investigations.