Olanzapine overdose

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We are writing to comment on the recent letter reporting olanzapine overdose (Johal & Shelly. Anaesthesia 2000; 55: 929). The letter describes a typical case of olanzapine poisoning, but although there had been over 10 case reports already published at the time the letter would have been submitted, the authors have not referred to any of these cases and state that the effects of toxicity are not well defined. This is clearly not the case with both considerable data on the pharmacology of olanzapine and other similar antipsychotic drugs, and a significant number of cases of overdose reported to the manufacturer of the drug [1]. The toxic effects that Johal and Shelly suggest can be extrapolated from those of other similar drugs are incorrect, with only central nervous system (CNS) depression and coma being reported. In the case that the authors report, they describe the typical features of olanzapine overdose, with CNS depression, miosis and tachycardia. These have been reported in numerous case reports and case series already and do not add to our current understanding of the drug in overdose [1-3]. In the light of this understanding, any of the investigations done were unnecessary, including the clotting screen, creatine kinase and urine myoglobin. Although deaths have been reported [4], supportive care is usually all that is required in olanzapine overdoses. It is unclear in the authors' statement about the possible toxic effects of olanzapine what data these features were extrapolated from. The clinical effects can be extrapolated from the known receptor effects of olanzapine, past case reports of olanzapine overdose and clinical effects of other agents of its class (clozapine, quetiapine). Olanzapine acts at a number of receptors, which predict its effects in overdose quite accurately [5]. It has antimuscarinic activity causing tachycardia without arrhythmia, reduced bowel sounds, agitation and altered mental status; antihistaminic activity causing sedation; alpha-adrenergic effects causing miosis, orthostatic hypotension and tachycardia, and agitation [3]. This is supported by previous case reports and case series of olanzapine overdoses [1-3] and also by clozapine overdoses [6], a similar drug that has been used for many years. This differs markedly from the effects listed by the authors, which are actually more typical of the older antipsychotic agents. Importantly, convulsions, QT-prolongation and AV block have not been reported with olanzapine and related antipsychotic drugs, and are not likely to occur.