Adverse effects commonly reported with Mirtazapine are increase in appetite and weight; drowsiness or sedation generally occurs during the first few weeks of treatment.
Dizziness, headache, edema and increase in liver enzyme levels have been reported less commonly; jaundice may occur. Other rarely reported adverse effects include postural hypotension, exanthema, nightmares, agitation, mania, hallucinations, paraesthesia, convulsions, tremor, myoclonus, restless leg syndrome, arthralgia, myalgia, and reversible agranulocytosis, leucopenia, and granulocytopenia.
Hyponatremia, possibly due to inappropriate secretion of antidiuretic hormone, has been associated with the use of antidepressants, particularly in the elderly.
Akathisia that developed in 2 patients given Mirtazapine 30 mg at night resolved in one after being treated with Clonazepam and in the other patient after reducing the dose of Mirtazapine to 15 mg at night.
The serotonin syndrome is most commonly due to the additive adverse effects of two or more drugs that enhance serotonin activity at central receptors; rarely, a single serotonergic drug has caused the syndrome. One such case occurred in an elderly patient given Mirtazapine 15 mg daily; he was also taking Salbutamol, Ipratropium and Nimodipine; although none of these are known to have serotonergic effects.