Managing schizophrenia can be incredibly challenging. Drug interactions are a major factor in that challenge. Olanzapine is broken down by CYP1A2. Smoking causes induction of the CYP1A2 enzyme. The olanzapine smoking cessation interaction is one that we should educate our patients about. When CYP1A2 is inhibited, this would lead to higher concentrations of olanzapine. When CYP1A2 is induced, it would lead to a reduction in olanzapine concentrations. There are also many other drug interactions that you need to be aware with smoking cessation. For today, we are going to stick to the olanzapine smoking cessation interaction. Here’s the case;
A 28 year old male patient with schizophrenia is being treated with olanzapine 15 mg twice daily. He has been fairly well controlled and the dose has been stable for at least 6 months. His primary provider has been working with him to try to get him to stop smoking. He has not been smoking cigarettes for approximately 1 week. He is on varenicline to help with smoking cessation.
The patient is now reporting an increase in tremor and also feels very sedated. A CBC and thyroid function testing was done to help rule out the cause of the fatigue. Both were negative for any concerns. The PCP believed that the varenicline was the cause of the new onset fatigue and tremor. Varenicline was stopped and the symptoms persisted.
I always recommend reviewing any type of change in a patient’s routine. Changes in lifestyle, supplements, over-the-counter medications, or prescription medications can lead to new changes. In this scenario, stopping smoking allowed for the increased accumulation and side effects from the olanzapine. In schizophrenia, it is highly important to recognize that smoking changes can significantly alter concentrations of olanzapine and clozapine.
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Smoking also interferes with the pharmacokinetics of vancomycin. Trough levels in a smoker will be much less than predicted.
When dosing I always take into account whether or not the patient is a smoker.