Lumefantrine D18 is primarily used as an internal standard in analytical chemistry for the quantification of lumefantrine in biological samples. It plays a critical role in ensuring the accuracy and precision of mass spectrometry-based methods, such as LC-MS/MS, during drug metabolism and pharmacokinetic studies. By serving as a stable isotope-labeled reference, it helps correct for variations in sample preparation and instrument performance, enabling reliable measurement of lumefantrine levels in plasma, blood, or other matrices. This is particularly important in clinical research and therapeutic drug monitoring for antimalarial treatments, where precise dosing and efficacy assessments are essential.