Bupropion D9 is primarily used as an internal standard in analytical chemistry, particularly in mass spectrometry and chromatography. It aids in the accurate quantification of bupropion and its metabolites in biological samples, such as blood or urine. This application is crucial in pharmacokinetic studies, where researchers assess drug absorption, distribution, metabolism, and excretion. Additionally, it plays a role in forensic toxicology to detect and measure bupropion levels in cases of overdose or misuse. Its stable isotopic labeling ensures precision and reliability in these analyses, making it an essential tool in both clinical and research settings.