An antidepressant (NaSSA)
Mirtazapine
An antidepressant that also aids sleep and appetite — often chosen when poor sleep, anxiety or weight loss are prominent, and taken at night.
What is Mirtazapine?
Mirtazapine treats depression, and is often chosen when sleep problems, anxiety or poor appetite are prominent because it tends to be sedating and can increase appetite. It is usually taken at night. Its effect on mood builds over a few weeks. Common effects are drowsiness, increased appetite and weight gain, which are often more noticeable early on.
Education and reference only. This is a plain-language guide to Mirtazapine — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Mirtazapine is an antidepressant that works differently from the common SSRIs. It is frequently chosen for depression when a person also has trouble sleeping, marked anxiety, or is losing weight and appetite, because it tends to be calming, aids sleep and stimulates appetite. It is sometimes added to another antidepressant under specialist guidance. It is taken as a once-daily tablet (including a version that melts on the tongue), usually at bedtime, and its full mood benefit takes a few weeks to appear.
How it works
Mirtazapine increases the activity of two of the brain’s mood chemicals, noradrenaline and serotonin, but by a different route from SSRIs — it blocks certain "brake" receptors so that more of these transmitters are released, and it blocks specific serotonin and histamine receptors. Blocking histamine is what makes it sedating and appetite-stimulating, especially at lower amounts, while the overall effect on noradrenaline and serotonin lifts mood over several weeks.
What it treats
Conditions Mirtazapine is used for
Practical use
How to take Mirtazapine
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it once a day at bedtime, as it usually causes drowsiness.
- Allow a few weeks for the mood benefit to build; keep taking it even if you do not feel a change at first.
- Do not stop suddenly — it is reduced gradually to avoid withdrawal effects.
- Report any worsening mood, agitation or distressing thoughts in the first weeks, especially if you are a young adult.
- Report fever, sore throat or mouth ulcers promptly, as (rarely) it can affect the blood count.
Weighing it up
Advantages & disadvantages of Mirtazapine
Advantages
- Helps sleep and appetite, so it suits depression with insomnia, anxiety or weight loss.
- Fewer sexual side effects and less stomach upset than many SSRIs.
- Once-daily, with a tablet that melts on the tongue for those who prefer it.
Disadvantages
- Commonly causes drowsiness, increased appetite and weight gain.
- Takes weeks to work and must be tapered when stopping.
- Rarely affects the blood count.
Practical use
Good to know
Mirtazapine is usually taken at night because it tends to make you drowsy — and, perhaps surprisingly, the sedating effect can be stronger at lower amounts than higher ones. Increased appetite and weight gain are common and are sometimes a helpful side effect when depression has caused weight loss, but unwelcome otherwise. Like all antidepressants, it should not be stopped abruptly (to avoid withdrawal effects), the mood benefit takes a few weeks, and there can be a temporary increase in anxiety or, particularly in younger adults, in distressing thoughts in the early weeks — which is why early follow-up matters. Rarely it can affect the blood count; report a fever or sore throat.
Who should not take it / use with caution
- People taking a monoamine oxidase inhibitor (MAOI) antidepressant, or within a washout period of one.
- Used with care in significant liver or kidney disease, epilepsy, heart disease, and certain blood conditions.
- Pregnancy and breastfeeding — considered individually, balancing benefit and risk.
Monitoring
- Mood and any suicidal thoughts, especially early and in younger adults
- Weight
- Blood count if signs of infection; sodium in older people
Side effects
- Common: drowsiness, increased appetite and weight gain, dry mouth.
- Sometimes dizziness, headache, vivid dreams, or swelling of the ankles.
- Rarely, a fall in white blood cells (report fever/sore throat), or a low sodium level, especially in older people.
Key interactions
- MAOIs must not be combined (serotonin syndrome risk) — a washout gap is needed.
- Alcohol and other sedating medicines add to drowsiness.
- Combining with other serotonin-raising medicines can, uncommonly, contribute to serotonin syndrome; enzyme-inducing or -inhibiting drugs can change its level.
Available as: Tablets and orodispersible ("melt-in-the-mouth") tablets.
Answers
Mirtazapine: frequently asked questions
Why do I take mirtazapine at night?
Mirtazapine tends to cause drowsiness, so taking it at bedtime uses that to help sleep and reduces daytime sleepiness. Interestingly, the sedating effect can be more pronounced at lower amounts, so do not assume a higher dose will be more sedating.
Will mirtazapine make me gain weight?
It commonly increases appetite and can lead to weight gain, which is sometimes helpful when depression has caused weight and appetite loss, but unwelcome otherwise. If weight gain is a concern, discuss it with your prescriber — attention to diet and activity helps, and an alternative can be considered.
How long until it works?
The effect on sleep and appetite can be felt quickly, but the improvement in mood usually takes two to four weeks or more to build. Keep taking it consistently, and stay in touch with your prescriber, especially in the first few weeks.
Can I stop it once I feel better?
Not suddenly. Antidepressants are usually continued for a period after you feel well to prevent relapse, and mirtazapine is reduced gradually rather than stopped abruptly to avoid withdrawal effects. Any change should be planned with your prescriber.
Authoritative sources
- BNF: Mirtazapine.
- electronic Medicines Compendium (SmPC): Mirtazapine (Zispin).
- NICE NG222: Depression in adults – treatment and management.
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