Method and apparatus for monitoring a dosage of at least one drug

The present invention proposes a device for monitoring a dosage of at least one administered to a patient Medikamets, wherein the measured in the breathing gas of a patient of an active substance concentration of the drug is compared with a calculated concentration in the respiratory gas. When changing the applied dose of the drug, the concentration range (13), has within which the measured concentration in the breathing gas are calculated, is adjusted. The invention also proposes a device for applying at least one medicament, a method for monitoring a dosage and a method for treating a patient.