This paper investigates the issue of interference mitigation in body sensor networks (BSNs). IEEE 802.15 Task Group 6 presented several schemes to reduce interference, but these are still not proper solutions for BSNs. We present a novel distributed TDMA-based flexible beacon scheduling scheme that reduces interference among the BSNs. A design goal of the scheme is to avoid the wakeup period of each BSN coinciding with other networks by employing carrier sensing before a beacon transmission. We analyze the flexible beacon scheduling scheme and investigate the proper back-off length when the channel is busy. We compare the performance of the proposed scheme with the schemes of IEEE 802.15 Task Group 6 using an OMNeT++ simulation. The simulation results show that the proposed scheme has a lower packet loss, energy consumption, and delivery-latency than the schemes of IEEE 802.15 Task Group 6.