Mirtazapine may increase the CNS depressant effect of alcohol. Patients should therefore be advised to avoid alcoholic beverages while taking mirtazapine.
Mirtazapine should not be administered concomitantly with MAO inhibitors or within two weeks after discontinuation of MAO inhibitor therapy.
Mirtazapine may increase the sedating properties of benzodiazepines and other sedatives (notably most antipsychotics, antihistamine H1 antagonists, opioids). Caution should be exercised when these medicinal products are prescribed together with mirtazapine.
Mirtazapine dosed at 30 mg once daily caused a small but statistically significant increase in the international normalized ratio (INR) in subjects treated with warfarin. As at a higher dose of mirtazapine a more pronounced effect cannot be excluded, it is advisable to monitor the INR in case of concomitant treatment of warfarin with mirtazapine.
The risk of QT prolongation and/or ventricular arrhythmias (e.g. Torsade de Pointes) may be increased with concomitant use of medicines which prolong the QTc interval (e.g. some antipsychotics and antibiotics).
The enzyme inhibitor drug metabolism CYP3A4, CY2D6 and CYP1A2 (cimetidine, azole antifungal derivatives, viral HIV-protease inhibitor drugs, erythromycin) increase the concentration in the blood and may increase the toxicity of mirtazapine. Conversely, carbamazepine and other inducers of CYP3A4 increased the clearance of mirtazapine and the dose of mirtazapine may need to be increased when given with these drugs.