The effective dose is an important tool in the radiation protection community because it represents the health risk associated with different procedures involving ionizing radiation, and therefore it allows comparing them. Therefore, accurate determination of the effective dose for nuclear medicine procedures is important. In this study, the effective dose per unit activity administered was calculated for some of the 99mTc-based radiopharmaceuticals. The MIRD method was used for the calculation of organ’s absorbed doses using the ICRP 110 adult male and female reference phantoms. The biokinetic data was taken from the ICRP Publications 128 and 53. Then, the effective doses were calculated using the ICRP 103 tissue weighting factors. The results show that with some exceptions, the calculated effective doses using new phantoms and tissue weighting factors are lower than the ICRP published data. This reduction is significant in some cases and can significantly reduce the collective effective dose of patients.