Updated: January 23, 2026

Mirtazapine Side Effects: What to Expect and When to Call Your Doctor

Q: Does mirtazapine cause weight gain?

Yes — weight gain is one of mirtazapine's most common side effects. Clinical studies show significant increases in appetite and weight, particularly in the first few months of treatment. This is due to blockade of H1 and 5-HT2C receptors. Weight gain can range from a few pounds to more significant amounts with long-term use.

Q: Why is mirtazapine more sedating at lower doses?

At doses of 7.5-15 mg, mirtazapine's strong H1 receptor blockade dominates, producing pronounced sedation. At higher doses (30-45 mg), the drug's noradrenergic activation partially offsets the H1 sedation, making patients feel less drowsy. This is why the medication is paradoxically 'more activating' at higher doses.

Q: Does mirtazapine cause sexual side effects?

Compared to SSRIs, mirtazapine has a relatively favorable sexual side effect profile. It is less likely to cause decreased libido, delayed orgasm, or erectile dysfunction than SSRIs. This makes it a common alternative for patients who experienced sexual dysfunction on SSRI therapy.

Q: What are the signs of agranulocytosis from mirtazapine?

Agranulocytosis is a rare but serious drop in white blood cells. Warning signs include unexplained fever, chills, sore throat, mouth sores, or unusual infections that don't resolve. If you experience these symptoms while taking mirtazapine, stop the medication and contact your doctor or emergency services immediately.

Author

Peter Daggett

Summarize with AI

The Boxed Warning: Suicidal Thoughts
Most Common Side Effects (>10% of Patients)
Less Common Side Effects (<10% of Patients)
Serious Side Effects: Call Your Doctor or Seek Emergency Care
Tips to Manage Common Side Effects
A Note on Sexual Side Effects

Learn the most common and serious mirtazapine (Remeron) side effects, how to manage them, and which symptoms require immediate medical attention.

Mirtazapine (Remeron) has a side effect profile that sets it apart from most other antidepressants. Some of its "side effects" are actually used therapeutically — like sedation and appetite stimulation. Understanding what to expect helps you manage your medication effectively and recognize when something needs medical attention.

The Boxed Warning: Suicidal Thoughts

Like all antidepressants, mirtazapine carries a FDA black box warning for increased risk of suicidal thoughts and behaviors in children, adolescents, and young adults under age 24. This risk is highest in the first few weeks of treatment and during dose adjustments. Mirtazapine is not FDA-approved for patients under 18.

If you or someone you know experiences new or worsening thoughts of self-harm while on mirtazapine, seek emergency help immediately or call/text 988 (Suicide & Crisis Lifeline).

Most Common Side Effects (>10% of Patients)

The following side effects are reported in more than 10% of patients taking mirtazapine:

Somnolence/sedation (54%): The most common side effect. At lower doses (7.5-15 mg), mirtazapine is strongly sedating due to H1 receptor blockade. This often improves at higher doses (30-45 mg) as noradrenergic effects increase.

Increased appetite and weight gain: Mirtazapine stimulates appetite via H1 and 5-HT2C blockade. Clinical trials showed 49% of pediatric patients and significant proportions of adults gained weight. Average weight gain is 3-5 lbs in the first few months.

Dizziness: Especially when standing up quickly (orthostatic hypotension). Taking mirtazapine at bedtime reduces daytime dizziness.

Dry mouth: Common; usually mild and manageable with fluids, sugar-free gum, or dry mouth rinses.

Constipation: Reported in many patients; usually manageable with dietary fiber and adequate hydration.

Fatigue/asthenia: A feeling of weakness or general tiredness, particularly in the first few weeks.

Less Common Side Effects (<10% of Patients)

Vivid dreams or nightmares

Restless legs syndrome (reported in 8-28% of patients in some sleep studies)

Increased cholesterol and triglycerides

Elevated liver enzymes (transient; rarely serious)

Hyponatremia (low sodium), especially in elderly patients

Tremor (less common than with SSRIs)

Serious Side Effects: Call Your Doctor or Seek Emergency Care

The following are rare but serious. Seek immediate medical attention:

Agranulocytosis: Dangerously low white blood cell count. Signs include fever, sore throat, mouth sores, or unusual infections. Call your doctor immediately.

Serotonin syndrome: Agitation, fast heart rate, high fever, muscle rigidity, diarrhea. More likely when combined with other serotonergic drugs (MAOIs, triptans, tramadol).

DRESS (Drug Reaction with Eosinophilia and Systemic Symptoms): A rare, potentially life-threatening skin reaction. Signs include rash, swollen lymph nodes, fever. Stop mirtazapine and seek emergency care.

QT prolongation / Torsades de Pointes: Rare heart rhythm abnormality. Signs include heart palpitations, fainting. Risk is higher in overdose or with concomitant QT-prolonging drugs.

Mania/hypomania: Mirtazapine can trigger manic episodes in patients with undiagnosed or diagnosed bipolar disorder. Signs include elevated mood, racing thoughts, decreased need for sleep.

Angle-closure glaucoma: Sudden eye pain, vision changes, or redness. Seek immediate eye care.

Tips to Manage Common Side Effects

Sedation: Take mirtazapine at bedtime. Many patients find sedation improves after 1-2 weeks as the body adjusts.

Weight gain: Maintain regular exercise and mindful eating. Discuss with your prescriber if weight gain is significant.

Dry mouth: Drink water regularly, use sugar-free gum, and consider dry mouth rinses.

Constipation: Increase dietary fiber and water intake. Stool softeners may help if needed.

A Note on Sexual Side Effects

Compared to SSRIs, mirtazapine has relatively low rates of sexual dysfunction. Research suggests it's less likely to cause decreased libido, erectile dysfunction, or delayed orgasm than selective serotonin reuptake inhibitors. This is one reason it's sometimes chosen for patients who experienced sexual side effects on SSRIs.

For more on mirtazapine safety, read our guide: Mirtazapine drug interactions: what to avoid.

If you're having difficulty finding your mirtazapine prescription, medfinder can help you locate it at a nearby pharmacy.

Frequently Asked Questions

Yes — weight gain is one of mirtazapine's most common side effects. Clinical studies show significant increases in appetite and weight, particularly in the first few months of treatment. This is due to blockade of H1 and 5-HT2C receptors. Weight gain can range from a few pounds to more significant amounts with long-term use.

At doses of 7.5-15 mg, mirtazapine's strong H1 receptor blockade dominates, producing pronounced sedation. At higher doses (30-45 mg), the drug's noradrenergic activation partially offsets the H1 sedation, making patients feel less drowsy. This is why the medication is paradoxically 'more activating' at higher doses.

Compared to SSRIs, mirtazapine has a relatively favorable sexual side effect profile. It is less likely to cause decreased libido, delayed orgasm, or erectile dysfunction than SSRIs. This makes it a common alternative for patients who experienced sexual dysfunction on SSRI therapy.

Agranulocytosis is a rare but serious drop in white blood cells. Warning signs include unexplained fever, chills, sore throat, mouth sores, or unusual infections that don't resolve. If you experience these symptoms while taking mirtazapine, stop the medication and contact your doctor or emergency services immediately.

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