Used as a stable isotope-labeled internal standard in analytical chemistry, particularly in mass spectrometry and gas chromatography for the accurate quantification of p-cresol in biological samples such as urine and plasma. Its deuterated methyl group provides a distinct mass shift, enabling clear differentiation from non-labeled compounds during detection. This application is critical in metabolic studies, especially in monitoring uremic toxins in patients with renal dysfunction and in research on gut microbiota metabolism. Due to its isotopic purity and chemical similarity to endogenous p-cresol, it ensures high precision and reliability in pharmacokinetic and biomarker validation studies.