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Reduce antipsychotic dose ‘by half’ in non-smokers, recommend researchers

GPs should use lower doses of antipsychotic drugs in patients who do not smoke, recommend researchers.

The meta-analysis – published in BMJ Open this month – recommend that the dose of certain antipsychotic drugs could be reduced by as much as 50% when prescribing them to patients who do not smoke.

The analysis looked at the effects of smoking on plasma levels of olanzapine or clozapine, relative to the doses prescribed.

For olanzapine, the concentration-to-dose ratio was around 0.75 ng/mL higher in smokers, compared with non-smokers. This would mean, for example, a daily dose of 10 mg given to patients who smoke should be cut by 30% to 7 mg in non-smokers.

For clozapine the difference in concentration-to-dose ratio was even greater, at around 1.11 ng/mL, meaning a daily dose of 200 mg should be halved to 100 mg.

The Japanese study authors, from Hirosaki University, noted clozapine in particular has been linked to increased incidences of serious central nervous system side effects after patients stopped or cut down on smoking.

They concluded: ‘We suggest that the doses of olanzapine and clozapine should be reduced by 30% and 50%, respectively, in non-smokers compared with smokers in order to obtain an equivalent olanzapine or clozapine.’

BMJ Open 2014; available online 4 March


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