Drug therapy is an essential part of pain treatment. However, only a minor part of pain patients benefits from the available treatments or is able to tolerate the drugs. One important limitation of drug therapy is lack of instruments to predict their effect. Indeed, in clinical practice "classes" of drugs (e.g. antidepressants) are given to "classes" of patients (e.g. neuropathic pain patients). However, within those classes of patients very different pain mechanisms are likely to underlie the pain condition in different patients. If drugs affect part of these mechanisms, they will not work in all patients. Another reason for variability in drug responses is genetic variation leading to a spectrum of different responses to analgesics, from lack of efficacy to exaggerated responses, up to intolerable adverse effects.