This paper presents a novel transmission power control protocol for body area networks. Conventional transmission power control protocols adjust the transmission power on the basis of the received signal strength indication (RSSI). However, in case of the presence of interference, the RSSI is not a correct indicator to determine the link state. We first present the empirical evidence for this and then propose a practical protocol to discriminate between the signal attenuation and interference using the RSSI and link quality indication (LQI). This protocol controls the transmission power and avoids interference based on the link state. Finally, we discuss the implementation of the proposed protocol on Tmote Sky and evaluate the performance in the presence and absence of interference. The experimental results showed that the proposed protocol has high energy-efficiency and reliability, even in the presence of interference.