Lamivudine, marketed under trade names such as Epivir® and Epivir-HBV®, is a nucleoside analog reverse-transcriptase inhibitor (NRTI) used for the treatment of human immunodeficiency virus (HIV) and hepatitis B. Therapeutic drug monitoring (TDM) of lamivudine concentrations in patient serum is performed to evaluate toxicity, guide dosing regimens, and avoid adverse drug-drug interactions. This Certified Spiking Solution® is suitable for use as starting material in the preparation of calibrators, controls, or linearity standards for a variety of LC-MS/MS or GC/MS lamivudine testing applications.