Medical implants with communication capability are becoming increasingly popular with today’s trends to continuously monitor patient’s condition. This is a major challenge for antenna designers since the implants are inherently small and placed in a communication-wise very lossy environment. Our goal is to determine the fundamental limitations of such antennas when placed inside human bodies and to develop guidelines for most efficient design. We base our findings on in-house analysis tools based on spherical and cylindrical wave expansion applied to simplified spherical and cylindrical body models respectively. These give us insight into wave propagation and show the maximum power density levels that can be reached just outside the body. Based on the obtained limits we can propose a useful upper bound for more complex scenarios.