Case reports of overdose with as much as 30 to 50 times the standard dose described the drug as relatively nontoxic, compared to tricyclic antidepressants.Twelve reported fatalities have been attributed to mirtazapine overdose. The fatal toxicity index (deaths per million prescriptions) for mirtazapine is 3.1 (95% CI: 0.1 to 17.2). This is similar to that observed with SSRIs. Interactions
Concurrent use with inhibitors or inducers of the cytochrome (CYP) P450 isoenzymes CYP1A2, CYP2D6, and/or CYP3A4 can result in altered concentrations of mirtazapine, as these are the main enzymes responsible for its metabolism. As examples, fluoxetine and paroxetine, inhibitors of these enzymes, are known to modestly increase mirtazapine levels, while carbamazepine, an inducer, considerably decreases them. Liver impairment and moderate chronic kidney disease have been reported to decrease the oral clearance of mirtazapine by about 30%; severe kidney disease decreases it by 50%.Mirtazapine in combination with an SSRI, SNRI, or TCA as an augmentation strategy is considered to be relatively safe and is often employed therapeutically, with a combination of venlafaxine and mirtazapine, sometimes referred to as "California rocket fuel". Several case reports document serotonin syndrome induced by the combination of mirtazapine with other agents (olanzapine, quetiapine, tramadol and venlafaxine).
In contrast to mirtazapine, the selective serotonin reuptake inhibitors (SSRI), serotonin–norepinephrine reuptake inhibitors (SNRI), Monoamine oxidase inhibitors (MAOI), and some Tricyclic antidepressants (TCA) increase the general activity of the 5-HT2A, 5-HT2C, and 5-HT3 receptors leading to a host of negative changes and side effects, the most prominent of which including anorexia, insomnia, nausea, and diarrhea, among others. Its reduced incidence of sexual dysfunction (such as loss of libido and anorgasmia) could be a product of negligible binding to the serotonin transporter (as is generally the cause of sexual dysfunction with most SSRIs) and antagonism of the 5-HT2A receptors; however, Mirtazapines high affinity towards and inverse agonism of the 5-HT2C receptors may greatly attenuate those pro-sexual factors (as evidenced by the pro-sexual effects of drugs like m-CPP and Lorcaserin which agonize 5-HT2C receptors in a reasonably selective manner). As a result, it is often combined with these drugs to reduce their side-effect profile and to produce a stronger antidepressant effect.Mirtazapine does not have pro-serotonergic activity and thus does not cause serotonin syndrome. This is in accordance with the fact that it is not a serotonin reuptake inhibitor or monoamine oxidase inhibitor (MAOI), nor a serotonin receptor agonist. There are no reports of serotonin syndrome in association with mirtazapine alone, and mirtazapine has not been found to cause serotonin syndrome in overdose. However, there are a handful of case reports of serotonin syndrome occurring with mirtazapine in combination with serotonergic drugs like SSRIs, although such reports are very rare, and do not necessarily implicate mirtazapine as causative.