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In order to assess the sensitivities of the radiation characteristics and the compliance of ingestible wireless device (IWD) in human body due to the uncertainty and intersubject variability of dielectric properties of human body tissues, the specific absorption rate (SAR) and radiation characteristics of the IWD in two realistic human body models with changed and unchanged dielectric values are quantitatively compared using the finite-difference time-domain method. Simulations are carried out in 13 scenarios where the IWD is placed in the center positions of abdomens in the two body models at the operating frequency of 430 MHz with three orientations. Results show that the variation of radiation intensity near the surface of abdomen is around 2.5 dB within 20% variation of dielectric values. The maximum SAR values increase with the increase in conductivities of human body tissues and decrease with the increase in relative permittivities of human body tissues. A variation of up to 20% in conductivities and relative permittivities, alone or simultaneously, always causes a variation of SAR to be less than 10%. As far as the compliance of safety is concerned, the maxima of 1-g-averaged and 10-g-averaged SARs can reach 3.16 and 0.89 W/kg at the input power of 25 mW.