#### Case scenario A 19 year old student has a six month history of social withdrawal, and bizarre behaviour. He becomes suspicious of his fellow students and believes that his teachers are monitoring his activity on Facebook. He is heard laughing and shouting when he is alone. Paranoid psychosis is diagnosed, and an atypical antipsychotic drug is prescribed alongside other psychosocial interventions (a combination recommended by current guidelines from the National Institute for Health and Clinical Excellence (NICE)1 The “typical” (first generation) antipsychotic drugs (such as haloperidol, chlorpromazine, and trifluoperazine) have been used to treat schizophrenia since the 1950s. The “atypical” (second generation) antipsychotics (table 1⇓) were introduced into routine practice from the 1990s. Both classes are used in the acute phase of schizophrenia and related psychoses and for long term maintenance and prevention of relapse. Typical antipsychotic drugs have a range of adverse effects, notably extrapyramidal features such as dystonia, parkinsonism, akathisia, and tardive dyskinesia. Although atypical antipsychotics are less likely to cause extrapyramidal symptoms, they may also have important adverse effects, including metabolic effects.3 Some atypical antipsychotics are also licensed to treat mood disorders, and table 1⇓ summarises licensed indications in the United Kingdom. However, this article considers their use for the treatment of non-affective psychosis only. View this table: Table 1 Current licensed (UK) atypical antipsychotics, with their licensed indications and available formulations2 Antipsychotics are a pharmacologically heterogeneous group of compounds, but all act as D2 dopamine receptor antagonists, an action linked to their antipsychotic effect.4 While the term atypical originally referred to antipsychotic drugs that did not cause catalepsy in animal models, recent academic and marketing literature uses this term to refer to both clinical properties (reduced incidence of extrapyramidal effects) and chemical properties (receptor profile).5 6 5-HT2A antagonism and/or 5-HT1A agonism may be …