Mirtazapine

Some of the views expressed in the following notes on newly approved products should be regarded as preliminary, as there may have been limited published data at the time of publication, and little experience in Australia of their safety or efficacy. However, the Editorial Executive Committee believes that comments made in good faith at an early stage may still be of value. Before new drugs are prescribed, the Committee believes it is important that more detailed information is obtained from the manufacturer's approved product information, a drug information centre or some other appropriate source.

Remeron (Organon)
Avanza (British Pharmaceuticals)
30 mg tablets
Approved indication: major depression
Australian Medicines Handbook Section 18.1

Mirtazapine is a tetracyclic antidepressant which was approved for marketing back in 1996. By antagonising central adrenoceptors, mirtazapine increases the release of noradrenaline and serotonin. As mirtazapine blocks 5HT2 and5HT3 receptors, the serotonin acts at 5HT1 receptors.1

The tablets have a bioavailability of 50% and peak plasma concentrations are reached two hours after a dose. Mirtazapine is extensively metabolised and has an average half-life of 20-40 hours. It is suitable for once daily dosing with a steady state being reached in three or four days. Clearance may be reduced by hepatic or renal impairment.

Treatment begins with 15 mg daily. If there is no response within 2-4 weeks, the dose can be increased. If there is no response to the maximum dose of 45mg, treatment should be stopped.

Several studies, many of them of only a few weeks' duration, have compared mirtazapine to placebo. Overall, mirtazapine is more effective. It has also been compared to other antidepressants. Most of these studies found the efficacy of mirtazapine to be statistically equivalent to amitriptyline, clomipramine, doxepin and trazodone. There are two published studies which suggest that mirtazapine has similar effects to fluoxetine and citalopram.

Mirtazapine is better tolerated than amitriptyline. Drowsiness can occur in the first few weeks of treatment and does not respond to reducing the dose. Other adverse effects include altered liver enzymes, bone marrow depression, oedema and weight gain. Although mirtazapine has weak anticholinergic activity, caution is advised when prescribing to patients with glaucoma or those at risk of urinary retention.

Mirtazapine may potentiate the effects of alcohol and benzodiazepines. The drug should not be prescribed with, or within two weeks of ceasing, a monoamineoxidase inhibitor.

References

  1. Mylecharane EJ. Serotonin receptor agonists and antagonists- mechanisms of action. Aust Prescr 1991;14:46-9.