Used as a stable isotope-labeled internal standard in quantitative analytical methods, particularly in liquid chromatography-mass spectrometry (LC-MS/MS) for the accurate measurement of sulfamethoxazole in biological samples such as plasma, serum, and urine. Its presence in methanol allows for direct injection into analytical systems without prior solvent evaporation or reconstitution, improving workflow efficiency. This compound ensures high precision in pharmacokinetic studies, therapeutic drug monitoring, and residue analysis in veterinary or environmental samples by compensating for matrix effects and instrument variability.