fluoxetine and olanzapine

Fluoxetine is a potent inhibitor of the cytochrome P450 enzyme CYP2D6, which is one of the pathways that metabolises olanzapine. Inhibition of this enzyme reduces the clearance of olanzapine, leading to increased plasma concentrations. Additionally, both drugs have central nervous system depressant effects that may be additive.

Elevated olanzapine levels increase the risk and severity of dose-dependent adverse effects, including sedation, dizziness, orthostatic hypotension, weight gain, hyperglycaemia, and extrapyramidal symptoms. There is also a potential for increased QTc prolongation. However, the combination is sometimes used intentionally to treat bipolar depression, and the effect can be therapeutically beneficial.

If the combination is initiated, consider a lower starting dose of olanzapine and monitor for signs of excessive sedation, weight gain, metabolic changes, and extrapyramidal symptoms. If fluoxetine is discontinued, monitor for reduced olanzapine efficacy and consider adjusting the olanzapine dose. Individualise treatment based on clinical response and tolerability.

The interaction is well established through pharmacokinetic studies showing significant increases in olanzapine exposure when co-administered with fluoxetine. This is consistent with the known CYP2D6 inhibitory profile of fluoxetine and the fact that the drugs are available in combination for bipolar depression.