Dielectric loss occurring in tissues in close proximity to UHF-implanted antennas is an important factor in the performance of medical implant communication systems. Common practice in numerical analysis and testing is to utilize radiation efficiency (RE) measures external to the tissue phantom employed. This approach means that RE is also dependent on the phantom used and antenna positioning, making it difficult to understand antenna performance and minimize near-field tissue losses. Therefore, an alternative methodology for determining the intrinsic radiation performance of implanted antennas that focuses on assessing structural and near-field tissue losses is presented. The new method is independent of the tissue phantom employed and can be used for quantitative comparison of designs across different studies. The intrinsic RE of an implant antenna is determined by assessing the power flow within the tissue phantom at a distance of at least λ g /2 from the radiating structure. The simulated results are presented for canonical antennas at 403 and 2400 MHz in homogeneous muscle and fat phantoms. These illustrate the dominance of propagating path losses in high-water content tissues such as muscle, whereas near-field dielectric losses may be more important in low-water tissues such as fat due to the extended reactive near-field.