Azaperone-d4 is primarily used in research and analytical studies as an internal standard in mass spectrometry. Its deuterated form allows for precise quantification and identification of azaperone and its metabolites in biological samples. This application is crucial in veterinary medicine for monitoring drug levels and ensuring proper dosing in animals. Additionally, it aids in pharmacokinetic studies to understand the absorption, distribution, metabolism, and excretion of azaperone, contributing to the development of safer and more effective veterinary treatments.