Used primarily as a stable isotope-labeled internal standard in analytical chemistry, especially in mass spectrometry and pharmacokinetic studies. It enables accurate quantification of flunixin in biological samples such as plasma, serum, and tissues by compensating for matrix effects and instrument variability. Commonly applied in veterinary drug monitoring and residue analysis due to flunixin's use in livestock. Its deuterated form ensures distinct mass differentiation while maintaining similar chemical behavior to the non-labeled compound, improving assay precision and reliability.