An osmotic pump includes a plurality of agent reservoirs; first and second solute chambers; a semi-permeable membrane separating the first and second solute chambers from one another; a flexible membrane separating a first one of the agent reservoirs from the first solute chamber; and a plurality of valves. Each of the valves is moveable between an open position in which a corresponding one of the agent reservoirs is open to an outlet of the pump and a closed position in which the corresponding one of the agent reservoirs is sealed with respect to the pump outlet.