An implantable medication delivery system comprising an implantable unit with a refillable reservoir, a catheter connected thereto, and a pumping mechanism activated by a microcomputer or microprocessor for pumping medication from the reservoir through the catheter into the body. The implantable medication unit receives information and control commands via a telemetry link from an external controller unit having a microprocessor. The external controller receives feedback in the form of intermittent sampling of blood using enzyme strips and a reflectance meter and/or additional sensor(s) which measure(s) physiological parameter(s) such as heart rate or blood pressure or temperature or skin resistivity. The feedback information is processed by the external unit in accordance with a mathematical model of the patient and the relevant parameters are transmitted to the implanted unit which adjusts its delivery profile according to a prescribed algorithm. The external unit can also detect an alarm condition and take appropriate steps, e.g. abort infusion.