Used as a stable isotope-labeled internal standard in mass spectrometry-based bioanalytical studies to accurately quantify lamivudine in biological samples such as plasma, serum, or urine. Its primary application is in pharmacokinetic and metabolism studies, where it helps ensure precise measurement of the drug concentration by compensating for matrix effects and instrument variability. It is essential in clinical research and drug development for antiretroviral therapies targeting HIV and hepatitis B virus (HBV).