Deuterium- and tritium-labeled pharmaceutical compounds are pivotal diagnostic tools in drug discovery research, providing vital information about the biological fate of drugs and drug metabolites. Herein we demonstrate that a photoredox-mediated hydrogen atom transfer protocol can efficiently and selectively install deuterium (D) and tritium (T) at α-amino sp³ carbon-hydrogen bonds in a single step, using isotopically labeled water (D₂O or T₂O) as the source of hydrogen isotope. In this context, we also report a convenient synthesis of T₂O from T₂, providing access to high-specific-activity T₂O. This protocol has been successfully applied to the high incorporation of deuterium and tritium in 18 drug molecules, which meet the requirements for use in ligand-binding assays and absorption, distribution, metabolism, and excretion studies.