Abstract: The present invention pertains to a non-invasive method and apparatus for predicting or monitoring analgesia and blood levels of opioid drugs in a patient receiving pain treatment, e.g., during palliative treatment. The inventive method comprises the measurement of one or more, preferably two or more, surrogate markers of a patient. According to the present invention surrogate markers correlate with the level of analgesia in the opioid receiving subject and thus provide a non-invasive method to predict and monitor analgesia during a treatment. Even more, surrogate markers were identified which correlate with the blood-concentration of the opioid in the subject. Thus, the invention provides a valuable clinical tool to assess and control pain treatments with opioids. Disclosed is the prediction method, an apparatus suitable for performing the inventive methods as well as the apparatus for use in medical treatments, such as pain therapy.