Energy consumption is an important issue in body area networks (BANs). In this letter, we propose an energy efficient transmission power control scheme for IEEE 802.15.6 BANs, which can improve energy efficiency by adaptively adjusting the transmit power in an on-demand way to adapt to varying channel environments. Simulations are performed to evaluate the performance, and it is shown that the proposed power control scheme outperforms traditional ones in terms of energy efficiency without significant reliability degradation.