Used as a stable isotope-labeled internal standard in analytical chemistry, particularly in mass spectrometry and liquid chromatography-mass spectrometry (LC-MS) methods. It helps in the accurate quantification of caffeine and related methylxanthines in biological samples such as blood, urine, and plasma. Due to its structural similarity to caffeine—differing only by deuterated methyl groups—it behaves almost identically during extraction and analysis, minimizing matrix effects and improving measurement precision. Commonly applied in pharmacokinetic studies, doping control, forensic toxicology, and metabolic research where reliable detection and quantification of caffeine metabolism are required.