An antidepressant that helps sleep and appetite — and the counterintuitive drug where a lower dose is more sedating.
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Mirtazapine is not an SSRI. It works differently — blocking particular receptors so that more noradrenaline and serotonin are released, while simultaneously blocking the serotonin receptors responsible for nausea and sexual dysfunction.
That mechanism explains its distinctive profile:
The counterintuitive part is the dose. Mirtazapine is more sedating at 15mg than at 45mg, because at higher doses the noradrenaline effect starts to offset the antihistamine effect. People who find 15mg unbearably drowsy are often told to increase rather than decrease, and it works.
It is not licensed as a sleeping tablet, and prescribing it purely for insomnia is off-label and not something we do — the sedation is a property, not the purpose.
If 15mg leaves you flattened the next morning, the answer is often to increase to 30mg rather than reduce. This genuinely surprises people, and it is the single most useful practical thing to know about the drug.
Sleep improves from the first few nights. The antidepressant effect still takes four to six weeks.
That early sleep benefit is real and valuable — but it is worth knowing that feeling better rested is not the same as the depression having lifted.
Care with: significant liver or kidney impairment, epilepsy, glaucoma, urinary retention, low blood pressure, and a history of blood disorders.
Particular care in older people, where sedation and dizziness increase falls risk, and where low sodium is more likely.
Weight is worth discussing honestly before starting. If weight is already a significant concern for you, mirtazapine is not the obvious first choice, and there are alternatives — that conversation is better had at the outset than a year later.
In pregnancy, less data exists than for SSRIs; sertraline is generally preferred where a choice is available.
Yes where clinically appropriate, and mirtazapine has a genuine place — particularly in depression where sleep and appetite are badly affected, or where SSRI sexual side effects have been the reason previous treatment failed.
What a consultation involves: full assessment including risk, an honest discussion of the weight gain, the dose paradox explained properly, and planned follow-up in the first few weeks.
The weight conversation is the one that matters. Mirtazapine causes more weight gain than most antidepressants, and it is frequently not mentioned at the point of prescribing. Someone who gains two stone over a year and was never warned has a legitimate grievance.
What we will not do: prescribe it as a sleeping tablet. The sedation is a side effect being put to use, not an indication, and using an antidepressant for insomnia without depression is not good practice.
What we will not manage remotely: severe depression with significant risk, psychosis, bipolar disorder, or anyone under 18.
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Mirtazapine is a cheap generic. On a private prescription it typically costs less than the England NHS prescription charge. NHS prescriptions are free in Wales.
The orodispersible tablets cost considerably more and are only worth it if you genuinely cannot swallow tablets. They are no more effective.
Therapy — free through NHS Talking Therapies, which takes self-referrals. And for the sleep problem specifically, CBT for insomnia has better long-term evidence than any sedating medication, and there are free NHS-endorsed digital programmes.
Weight gain. It is not a financial cost in the obvious sense, but new clothes and the health consequences of two stone are a real consideration, and worth weighing before starting rather than after.
Mirtazapine is not addictive, but stopping abruptly causes discontinuation symptoms — and one of them is distinctive: rebound insomnia, often severe, because the sedation you have relied on for sleep disappears overnight.
Also: nausea, anxiety, agitation, dizziness.
Because of how mirtazapine works. At 15mg the antihistamine sedation dominates. At 30mg and above, increased noradrenaline partly offsets it.
If 15mg leaves you flattened, increasing to 30mg often helps rather than making it worse. This is genuinely counterintuitive and is the most useful thing to know about the drug.
Quite possibly, and more than with most antidepressants. Increased appetite — particularly for carbohydrate — is very common, and gains of several kilograms over months are frequent.
It is the commonest reason people stop. Weigh yourself monthly rather than daily, and raise it early: switching is far easier than reversing a year of gradual gain.
No. It is an antidepressant that happens to be sedating, and prescribing it purely for insomnia is off-label. The sleep benefit is genuine, but it is not what the drug is for.
Far fewer than SSRIs, because of how it works on serotonin receptors. That is a real advantage, and it is why mirtazapine is sometimes chosen for people who stopped an SSRI for that reason.
Sleep improves within days. The depression takes four to six weeks, like any antidepressant. Sleeping better is welcome but is not the same as recovery.
Be careful. The combination is markedly sedating, more so than people expect, and it affects driving the following morning.
Yes — get a blood test. Mirtazapine can rarely suppress the bone marrow, and infection symptoms are how that presents. It is uncommon, and it is the one thing to act on rather than wait out.
Usually not. A first episode is typically treated for six to twelve months after recovery, then reduced gradually.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
September 5, 2026
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