Mirtazapine
Is It Right for You?
A complete guide to Mirtazapine — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Mirtazapine?
Mirtazapine is a noradrenergic and specific serotonergic antidepressant (NaSSA) with a distinct mechanism of action that sets it apart from SSRIs and SNRIs. Rather than blocking the reuptake of neurotransmitters, mirtazapine works by blocking specific receptors that regulate their release and action, resulting in enhanced noradrenergic and serotonergic neurotransmission through a fundamentally different pharmacological route.
Developed in the 1990s, mirtazapine is particularly valued for its sedating properties — which are paradoxically greater at lower doses — its ability to stimulate appetite, and its lack of the sexual side effects commonly associated with SSRIs and SNRIs. These characteristics make it especially useful for patients with depression accompanied by significant insomnia, weight loss, or poor appetite, and for those who have not tolerated other antidepressants due to sexual dysfunction or gastrointestinal side effects.
Mirtazapine is available as standard tablets, orodispersible tablets (which dissolve on the tongue without water), and oral solution, making it accessible to patients who have difficulty swallowing conventional tablets. It is taken once daily, typically at bedtime, and is suitable for adults.
Mirtazapine is a prescription-only medication in the UK. A licensed clinician must assess your symptoms and medical history before it can be prescribed — which can now be done quickly and conveniently through a telehealth consultation.
What Conditions Is Mirtazapine Used For?
Mirtazapine is licensed for:
including depression with prominent anxiety, insomnia, poor appetite, or agitation.
particularly effective where both depressive and significant anxiety symptoms coexist.
frequently used as augmentation therapy alongside an SSRI or SNRI when monotherapy has been insufficient.
well tolerated in this population and useful where sedation and appetite stimulation are beneficial.
While mirtazapine's sedating and appetite-stimulating effects are often felt within the first few days, full antidepressant effects typically develop over 2 to 4 weeks, with continued improvement over the following months. Do not discontinue early if mood has not yet lifted.
How Does Mirtazapine Work?
Mirtazapine enhances noradrenergic and serotonergic neurotransmission through a multi-receptor blocking mechanism:
Alpha-2 Adrenergic Receptor Antagonism — Mirtazapine blocks central presynaptic alpha-2 autoreceptors and heteroreceptors. These receptors normally act as "brakes" on neurotransmitter release — when blocked, they can no longer reduce the release of norepinephrine and serotonin, resulting in increased release of both neurotransmitters.
Selective Serotonin Receptor Blockade — Mirtazapine blocks 5-HT2A, 5-HT2C, and 5-HT3 receptor subtypes. By blocking 5-HT3 receptors (which mediate nausea and gastrointestinal side effects), mirtazapine avoids the nausea, diarrhoea, and sexual dysfunction commonly associated with SSRIs. By blocking 5-HT2C receptors, it stimulates appetite and weight gain. 5-HT2A blockade contributes to anxiolytic and sleep-promoting effects.
Histamine H1 Receptor Antagonism — Mirtazapine's potent antihistaminic activity is responsible for its marked sedation and appetite stimulation. This effect is paradoxically greater at lower doses, because the sedating H1 antagonism predominates before the activating noradrenergic effects become significant at higher doses.
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Take mirtazapine at bedtime — its sedating properties are significant, particularly at lower doses, and are best used advantageously to improve sleep at night.
The paradox of dose and sedation: Mirtazapine is often more sedating at 15 mg than at 30 or 45 mg. This is because, at lower doses, the sedating antihistaminic effect predominates. At higher doses, the activating noradrenergic component becomes more prominent, partially offsetting the sedation. If patients find mirtazapine excessively sedating, increasing the dose (under clinician guidance) may paradoxically reduce daytime drowsiness.
Orodispersible tablets should be placed on the tongue and allowed to dissolve — they are useful for patients who have difficulty swallowing and do not require water.
Do not stop mirtazapine abruptly — taper the dose gradually under clinician supervision to avoid discontinuation effects.
Strictly avoid alcohol during mirtazapine treatment — mirtazapine markedly enhances the sedative effects of alcohol, potentially causing profound and dangerous CNS depression.
Side Effects of Mirtazapine
Mirtazapine's side-effect profile reflects its receptor-blocking mechanism. It is notably free of the nausea, diarrhoea, and sexual dysfunction commonly seen with SSRIs.
- Somnolence and sedation (very common, particularly at lower doses)
- Increased appetite and weight gain (common and often significant)
- Dry mouth
- Constipation
- Peripheral oedema
- Dizziness
- Vivid dreams
- Agranulocytosis and neutropenia — rare but potentially life-threatening reduction in white blood cells; patients should seek immediate medical attention if they develop unexplained fever, sore throat, mouth ulcers, or other signs of infection
- Serotonin syndrome — particularly when combined with other serotonergic drugs; agitation, fever, muscle rigidity, confusion
- Suicidal ideation — increased risk in patients under 25, particularly in the initial weeks
- Hyponatraemia — low sodium, particularly in elderly patients
- Seizures — rare
- Mania activation — in patients with undiagnosed bipolar disorder
Although rare, mirtazapine has been associated with agranulocytosis — a dangerous reduction in neutrophils that impairs the body's ability to fight infection. Patients should be advised to seek immediate medical attention if they develop a high fever, sore throat, mouth ulcers, or other signs of serious infection during treatment. A full blood count should be performed promptly in such cases.
Drug Interactions
Always disclose all prescription drugs, over-the-counter medicines, vitamins, and supplements to your clinician or pharmacist before starting mirtazapine.
Contraindicated; risk of life-threatening serotonin syndrome. A minimum 14-day washout after stopping an MAOI is required before starting mirtazapine; allow 14 days after stopping mirtazapine before starting an MAOI.
Mirtazapine markedly potentiates CNS depression; combination with alcohol can be extremely dangerous, causing profound sedation and respiratory depression. Alcohol must be avoided during treatment.
Increased risk of serotonin syndrome; monitor carefully if combination is clinically necessary.
Significantly reduce mirtazapine plasma concentrations; dose increase may be required.
Important Warnings
Mirtazapine has been rarely associated with agranulocytosis. Inform your clinician immediately and seek urgent medical attention if you develop unexplained fever, severe sore throat, mouth ulcers, or any signs of severe infection during treatment. Do not wait — agranulocytosis can be life-threatening if not treated promptly.
Antidepressants including mirtazapine have been associated with an increased risk of suicidal thoughts in young adults under 25 in the early weeks of treatment. Close monitoring during the first month and following any dose changes is essential.
Mirtazapine's potent sedating effects are dramatically amplified by alcohol. The combination can cause severe respiratory depression and loss of consciousness. Patients must avoid alcohol entirely during mirtazapine treatment.
Although mirtazapine's discontinuation syndrome is generally less severe than that of paroxetine or venlafaxine, abrupt cessation can still cause nausea, dizziness, anxiety, and insomnia. Always taper the dose gradually under clinician supervision.
Speak to a Clinician About Treatment
Getting advice no longer means sitting in a waiting room. Through The GP Service, you can consult with a licensed clinician in minutes — from home, on your lunch break, or wherever works for you. If treatment is clinically appropriate, your clinician can issue a prescription during the consultation. A consultation does not guarantee a prescription.
The treatment you need, when you need it.
Mirtazapine FAQs
This is one of mirtazapine's most counterintuitive properties. At lower doses (7.5–15 mg), the dominant effect is potent histamine H1 receptor blockade, causing marked sedation and appetite stimulation. At higher doses (30–45 mg), noradrenergic activity becomes more prominent and partially counteracts the sedating antihistaminic effects. As a result, patients who find 15 mg excessively sedating may actually tolerate 30 mg better — a dose increase your clinician may recommend if daytime drowsiness is problematic.
Weight gain is a very common side effect of mirtazapine due to its appetite-stimulating and sedating properties. For patients with depression-related weight loss and poor appetite, this can be clinically beneficial. For patients who are already a healthy weight or overweight, weight management through diet and regular activity becomes important during treatment. Discuss this with your clinician before starting mirtazapine if it is a significant concern.
Mirtazapine is frequently prescribed off-label for insomnia, particularly in low doses (7.5–15 mg), where its sedating properties are most pronounced. It is particularly useful when insomnia coexists with depression, anxiety, or nausea. Its use for insomnia alone (without depression) should be discussed with your clinician.
No — this is one of mirtazapine's significant advantages over SSRIs and SNRIs. By blocking serotonin at 5-HT2 and 5-HT3 receptors rather than increasing serotonin broadly, mirtazapine does not cause the sexual dysfunction (reduced libido, delayed orgasm) commonly seen with SSRIs. It may even be prescribed specifically to patients experiencing SSRI-induced sexual dysfunction.
Mirtazapine works by an entirely different mechanism — blocking receptors that regulate neurotransmitter release, rather than blocking their reuptake. As a result, its side-effect profile is distinct: it causes sedation, weight gain, and appetite stimulation rather than nausea, insomnia, diarrhoea, and sexual dysfunction. It is often a preferred choice for patients who have not tolerated SSRIs or require specific benefits such as improved sleep and appetite.