A combination of two numerical techniques of computational electromagnetics, namely, method of moments and vector spherical wave expansion, is used to show performance limitations on the radiation efficiency of implantable antennas and to efficiently resolve computation difficulties imposed by the interaction of an electrically small radiator with its host body. The results computed for ideal and realistic radiation sources prove the significantly limited performance of implantable antennas. The role of substructure characteristic modes decomposition in the formulation of this fundamental limit is explained.