This paper investigates a method to improve performance of diffusive molecular communications between biologically-enabled nanomachines in in-vivo aqueous environment. The proposed method exploits periodic flow, e.g., induced by repeated heart pumping. We make an analysis of channel impulse response (CIR) for such drift-diffusion fluid systems. In order to take the cyclic CIR into account, the proposed method optimizes the release timing and size of information molecules so that highest equalization gain can be achieved. We reveal that error rate performance can be significantly improved with adaptive molecule loading by taking care of the cyclic CIR.