Dextromethorphan-d3 is primarily used as an internal standard in analytical chemistry, particularly in mass spectrometry. Its application is crucial for the accurate quantification of dextromethorphan in various biological samples, such as blood, urine, and plasma. By serving as a stable isotope-labeled analog, it helps correct for variations during sample preparation and analysis, ensuring more reliable and precise results. This is especially important in pharmacokinetic studies, where understanding the absorption, distribution, metabolism, and excretion of dextromethorphan is essential. Additionally, it aids in forensic and clinical toxicology to detect and measure dextromethorphan levels in cases of overdose or misuse. Its use enhances the accuracy and reproducibility of analytical methods, making it a valuable tool in research and diagnostic laboratories.