An arrangement is provided for monitoring a metering of at least one medication administered to a patient. The concentration of the active ingredient of the medication, which is measured in the breathing gas of a patient, is balanced with a computed concentration in the breathing gas. When a change of the administered quantity of the medication takes place, the concentration range (13) is adapted. The concentration, which is measured in the breathing gas, must lie by computation within this range. An arrangement for administering at least one medication and a method for monitoring a metering or dosing of the medication as well as a method for treating a patient are provided.