Despite its success in saving energy and increasing spatial reuse, the use of variable transmission power control in wireless multihop networks has consequences to fairness. We study a variant of the hidden terminal problem resulting from link asymmetries, and its impact on expected node throughput. We give a model relating the transmission power level and backoff window size to throughput. Based on the analysis, we propose a localized algorithm to set the contention window size of a medium access control protocol according to the transmission power level. In simulation our protocol achieves the highest fairness index among several approaches.