According to a first aspect of the present invention there is provided a controller for controlling a glucagon pump. The controller comprises an input 8 for receiving an input signal indicative of a measured blood glucose concentration, an output 9 for providing a pulsed glucagon pump drive signal, each pulse being arranged to cause a glucagon pump to deliver a volume of glucagon, and a processing unit 10 for setting the pulse frequency of the pump drive signal in dependence upon the input signal, such that when the input signal indicates that the measured blood glucose concentration is above a threshold concentration the pulse frequency is zero and when the input signal indicates that the measured blood glucose concentration is below the threshold concentration the pulse frequency is increased as the measured blood glucose concentration decreases.