Aspirin-d4 is primarily used as an internal standard in analytical chemistry for the quantification of aspirin and its metabolites in biological samples. It is particularly valuable in mass spectrometry and nuclear magnetic resonance (NMR) studies due to its deuterium labeling, which allows for precise tracking and differentiation from non-deuterated compounds. This makes it essential in pharmacokinetic and pharmacodynamic research, where accurate measurement of drug levels in plasma, urine, or tissues is critical. Additionally, it aids in studying the metabolic pathways and stability of aspirin in various environments, contributing to advancements in drug development and therapeutic monitoring.