💊 Mirtazapine
Drug Class & Overview
Mirtazapine is a noradrenergic and specific serotonergic antidepressant (NaSSA). It has a unique mechanism of action compared to SSRIs and SNRIs.
Mechanism of Action
Mirtazapine acts as an antagonist at presynaptic alpha-2 adrenergic autoreceptors and heteroreceptors, leading to increased release of noradrenaline and serotonin. It also blocks postsynaptic 5-HT2 and 5-HT3 receptors, and is a potent antagonist of histamine H1 receptors.
Key Indications
Mirtazapine is primarily indicated for the treatment of major depressive disorder. It is often considered for patients who experience insomnia, significant anxiety, or poor appetite/weight loss alongside their depression, due to its sedative and appetite-stimulating effects.
Contraindications
Absolute contraindications include concomitant use with MAO inhibitors due to the risk of serotonin syndrome. Caution is required in patients with epilepsy, liver or renal impairment, and those with a history of cardiac conditions. It should be used with caution in patients with angle-closure glaucoma or prostatic hypertrophy.
Adverse Effects
Common side effects include significant sedation, increased appetite and weight gain, and dizziness. Less common but serious effects include agranulocytosis (rare), orthostatic hypotension, and serotonin syndrome (when combined with other serotonergic drugs). Dry mouth and constipation can also occur.
Monitoring
Baseline full blood count should be considered due to the rare risk of agranulocytosis, with patients advised to report any signs of infection (e.g., fever, sore throat). Liver and renal function tests may be warranted in patients with pre-existing impairment. Regular monitoring for clinical response and adverse effects, particularly weight gain and sedation, is important.
Prescribing Safety (OSCE)
When prescribing mirtazapine, always check for drug allergies and potential interactions, especially with MAOIs and other CNS depressants. Counsel patients on the significant sedative effect, advising them to take it at bedtime and to avoid driving or operating machinery until they know how it affects them. Discuss the potential for weight gain and the importance of reporting any signs of infection.
MLA High-Yield Notes
Mirtazapine is often chosen for patients who have difficulty sleeping or have lost weight due to depression. Its H1 antagonism contributes significantly to its sedative properties. It has a relatively low risk of sexual dysfunction compared to SSRIs/SNRIs. The risk of agranulocytosis is very rare but important to remember for patient safety advice.
Common SBA Themes
SBA questions often highlight mirtazapine's unique side effect profile, particularly its sedative and appetite-stimulating properties, making it a good choice for depressed patients with insomnia or weight loss. Questions might also focus on its mechanism of action or the rare but serious risk of agranulocytosis and the associated patient counselling points.
References
- BNF (British National Formulary)
- NICE (National Institute for Health and Care Excellence) Guidelines
- MHRA (Medicines and Healthcare products Regulatory Agency) Guidance