Fundamental limits for implanted antennas should be able to evaluate the bounds on losses with the knowledge of physical limitations from antenna and tissue parameters. In this work, rules of thumb to assess losses of deeply implanted antennas are presented toward improving the radiation efficiency. By means of rigorous and approximate approaches, the radiation efficiency of an elementary antenna within a spherical phantom is calculated as a function of multiple variables. According to the rules summarized subsequently, losses from different contributions may play a key role in specific situations, such as changes in the antenna type, phantom size, implant location and operating frequency.