A medical apparatus for a patient comprises an implantable system. The implantable system comprises a first implantable device comprising: at least one implantable functional element configured to deliver stimulation energy to tissue of the patient; and an implantable controller configured to provide a stimulation waveform to the at least one implantable functional element, the stimulation waveform comprising one or more stimulation parameters. The apparatus is configured to randomly vary at least one of the one or more stimulation parameters. Methods of providing stimulation energy with randomly varying stimulation parameters are also provided.