Used as a stable isotope-labeled internal standard in mass spectrometry-based bioanalytical studies to accurately quantify lamivudine levels in biological samples such as plasma, serum, or urine. Its primary application is in pharmacokinetic and metabolism studies, where it helps differentiate between endogenous compounds and administered drug, ensuring high precision and reliability in drug monitoring and clinical research. Commonly employed in HIV and hepatitis B treatment studies to assess drug absorption, distribution, and clearance.