What are the most common mirtazapine side effects?
Mirtazapine commonly causes side effects that show up early and can change over time. Reported effects include sleepiness and fatigue, increased appetite, weight gain, dry mouth, dizziness, constipation, and changes in taste perception.[1][2]
Some people also notice effects related to mood and alertness, such as feeling drowsy during the day or being less able to concentrate.[1]
What serious side effects should you watch for?
Seek urgent medical help if you develop signs of a serious reaction, including:
- Allergic reactions such as swelling of the face/lips, trouble breathing, or widespread rash.[1]
- Severe dizziness or fainting that suggests low blood pressure.[2]
- Unusual bleeding or bruising, which can occur with antidepressants.[2]
- Signs of infection like fever or persistent sore throat with no clear cause, which can signal low white blood cells (rare).[1][2]
Mirtazapine can also (rarely) affect blood counts and cause more significant neurologic or immune-related problems; clinicians often advise reporting new or worsening symptoms promptly.[1][2]
Can mirtazapine cause weight gain or increased appetite?
Yes. Increased appetite and weight gain are among the best-known mirtazapine effects.[1][2] This can be noticeable within the first weeks to months, especially if appetite rises quickly.
If you’re concerned, a clinician may suggest dose timing changes, dietary strategies, or switching medications if weight gain becomes problematic.[1][2]
Does mirtazapine affect sleepiness or alertness?
Mirtazapine can make many people feel drowsy, and it is sometimes used when sleep problems are part of depression or anxiety. Daytime sleepiness can still happen, especially early in treatment or if the dose is higher.[1][2]
Avoid driving or operating machinery until you know how it affects you.[1]
What sexual side effects does mirtazapine cause?
Compared with some other antidepressants, mirtazapine is often less associated with sexual dysfunction, but it can still cause changes in sexual function in some people.[2] If this happens, discussing dose changes or alternative treatments with a clinician can help.
What about constipation, dry mouth, and dizziness?
Dry mouth and constipation are common, along with dizziness or lightheadedness.[1][2] Staying hydrated, using fiber for constipation, and rising slowly from sitting or lying down can reduce dizziness for some people.
What are withdrawal (stopping) symptoms like?
Stopping antidepressants suddenly can lead to discontinuation symptoms, such as dizziness, irritability, nausea, or flu-like feelings. If you want to stop or change therapy, it’s usually done with a clinician-guided taper rather than abruptly.[1][2]
When should you contact a doctor quickly?
Contact a clinician promptly for:
- Worsening mood, agitation, or new suicidal thoughts.[2]
- Symptoms of an allergic reaction or severe rash.[1]
- New severe headache, fever, confusion, or fainting.[1][2]
- Rapid weight gain or appetite increase that becomes hard to manage.[1][2]
How long do side effects last?
Many side effects improve after the first couple of weeks as your body adjusts, especially sleepiness and appetite-related changes. Others, like weight changes, may continue while you take the medication.[1][2] If side effects don’t ease or become intolerable, clinicians may adjust the dose or switch treatments.
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Sources
[1] https://www.nhs.uk/medicines/mirtazapine/
[2] https://www.mayoclinic.org/drugs-supplements/mirtazapine-oral-route/side-effects/drg-20067362