In this paper, we propose an interference management method for underlay spectrum sharing and evaluate its performance. In this method the secondary service has been facilitated by granting passive access to the power control signalling transmitted by the primary network base station. To exploit both slow shadowing and fast fading, the proposed method performs secondary service power management in two phases, each in different time-scales: rate optimal power allocation phase and interference reduction power adjustment phase. In the longer time-scale, rate optimal power allocation phase adaptively allocates the secondary transmit power exploiting the medium-scale channel variations (shadowing effect) of the secondary channel to maximize its capacity. In the shorter time-scale, interference reduction power adjustment phase exploits the power control commands transmitted in the primary network to adaptively adjust secondary service transmission power for reducing the effects of the secondary service transmission on the Quality-of-Service (QoS) of the primary service network. The main advantage of this method which is referred to as Adaptive Multiple Time-Scale Power Allocation (AMTPA) is that it does not require direct signaling between the two systems. We further present AMTPA analytical performance evaluation results. Practical considerations are also presented regarding the primary network requirements and its power control feasibility after adopting AMTPA in the secondary network. Extensive simulation results indicate significant improvement in the system performance by using AMTPA. Using simulations we also show how one can set AMTPA parameters so that a certain level of QoS in the primary network is satisfied.