Mirtazapine D3, a deuterated form of mirtazapine, is primarily used as an internal standard in analytical chemistry for the quantification of mirtazapine in various biological samples. Its application is critical in pharmacokinetic studies, where it helps ensure accurate and precise measurement of mirtazapine levels in plasma, serum, or urine. The deuterium labeling minimizes interference during mass spectrometry analysis, enhancing the reliability of results. Additionally, it aids in drug monitoring and research, particularly in understanding the metabolism, distribution, and elimination of mirtazapine in clinical and forensic settings. Its use supports the development of therapeutic drug monitoring protocols and ensures compliance and efficacy in patients undergoing treatment with mirtazapine.