A cannula assembly and a medical system. The cannula assembly includes a cannula, a capnometer, a reservoir, a pathway, and a barrier. The cannula is positionable on the face of a patient. The capnometer measures carbon dioxide gas concentration and is operably connected to the cannula. The reservoir is adapted for containing a known concentration of carbon dioxide gas. The pathway connects carbon dioxide gas in the reservoir with the capnometer. The barrier has a first state preventing gas flow along the pathway and has a second state allowing gas flow along the pathway. The medical system includes the cannula assembly and includes a drug delivery assembly. The drug delivery assembly is adapted for administering a drug to the patient according to a drug delivery schedule based at least in part on the carbon dioxide gas concentration of the exhaled air of the patient as measured by the capnometer.