A copper-catalyzed procedure enabling dynamic carbon isotope exchange is described. Utilizing the universal precursor [C-14]CO2, this protocol allows to insert, in one single step, the desired carbon tag into carboxylic acids with no need of structural modifications. Reducing synthetic costs and limiting the generation of radioactive waste, this procedure will facilitate the access to carboxylic acids containing drugs and accelerate early C-14-based ADME studies supporting drug development.