This communication presents a method for the fast and accurate estimation of the gain pattern and maximum gain of an implanted antenna including the effect of the host body, under the assumption that the latter is electrically large. The estimation procedure is based on the radiation of an elementary dipole source placed in a planar body model. The derivation of closed-form expressions is based on spherical wave analysis and Green's functions for layered media. The validity of this approximation for practical cases is shown on different implanted antennas, where the results are compared to full-wave simulations and measurements.