Doxepin-d3 Hydrochloride is primarily used in research and analytical applications as an internal standard in mass spectrometry. It aids in the accurate quantification and identification of doxepin and its metabolites in biological samples. This deuterated compound is valuable in pharmacokinetic studies, helping researchers understand drug metabolism and distribution in the body. It is also utilized in forensic toxicology to detect and measure doxepin levels in cases of overdose or poisoning. Its stable isotopic labeling ensures minimal interference, providing reliable and precise results in complex matrices.