A method of monitoring a concentration of an anesthetic drug using a patient's breath is provided. The method comprises forming a breath sample using the patient's breath; exposing one or more sensors to the breath sample; detecting one or more components of the anesthetic drug in the breath sample; measuring a concentration of at least one of the components of the anesthetic drug in the breath sample; and determining a concentration of the component in a plasma of the patient using a transfer function and the concentration of the component in the breath sample.