Fundamental bounds on achievable radiation efficiency serve as the design quality gauge, facilitate the choice of the antenna type considering the available dimensions, and provide simple rules to check the feasibility of a given design. This study quantifies the effect of body-implanted capsule dimensions and materials on achievable radiation efficiency. We also show that a dielectric-loaded electric antenna operating close to the optimal frequency can radiate more efficiently than a magnetic one. The latter, however, is more efficient when electrically small.