The atypical antipsychotic agent olanzapine is structurally similar to clozapine, and is used in the treatment of schizophrenia and related disorders. It has high affinity for dopamine D1, D2, D4, 5HT2A, 5HT2C, 5HT3, alpha1-adrenergic, histamine H1 and five subtypes of muscarinic receptors, and has been shown to improve both the positive and negative symptoms of schizophrenia. Olanzapine is extensively metabolised and distributed throughout the body, although the principal metabolites are thought to lack pharmacological activity. Dosage is normally 5-20 mg/d (BNF recommended limit), but some patients are given up to 60 mg/d. Dose-related side effects associated with olanzapine may include somnolence, weight gain, dizziness, peripheral oedema, and postural hypotension. Olanzapine assay can be useful to (i) assess adherence, (ii) assess adequacy of dosage, and (iii) investigate suspected acute poisoning. A steady-state reference range of 20 – 40 µg/L has been suggested (12h post-dose sample).