Used as a stable isotope-labeled internal standard in mass spectrometry and liquid chromatography-tandem mass spectrometry (LC-MS/MS) for the accurate quantification of creatine in biological samples such as urine, plasma, and tissues. Its deuterated methyl group allows clear differentiation from endogenous creatine, improving measurement precision in metabolic studies, doping control, and clinical diagnostics related to creatine metabolism disorders. Also employed in pharmacokinetic and bioavailability studies to track creatine uptake and turnover in muscle and brain tissue without altering the compound’s biological activity.