Mirtazapine side effects: a comprehensive guide

The most frequently reported side effects of mirtazapine are well-documented in the BNF and NHS patient information. Many are related to its potent antihistamine (H1) activity.

Very common (affecting more than 1 in 10 people):

• Increased appetite and weight gain: this is the most widely discussed side effect. Mirtazapine's antihistamine action stimulates appetite, and many patients gain weight, particularly in the first few months
• Drowsiness and sedation: most pronounced at lower doses (15 mg) due to proportionally greater H1 blockade. Paradoxically, higher doses may be less sedating as noradrenergic effects increase
• Dry mouth: affects a significant proportion of patients

Common (affecting 1 in 10 to 1 in 100 people):

• Fatigue and lethargy: distinct from drowsiness, this is a general lack of energy
• Headache
• Dizziness
• Oedema: fluid retention causing swelling of ankles and feet
• Constipation
• Abnormal dreams or vivid dreams

These side effects are a trade-off for mirtazapine's antidepressant benefit.

Many patients find that drowsiness improves within the first one to two weeks, while weight gain may persist throughout treatment.

Weight gain is arguably the most significant practical concern for patients taking mirtazapine. Understanding why it happens can help with management.

Why does mirtazapine cause weight gain?

• H1 receptor blockade: mirtazapine is one of the most potent antihistamines among antidepressants. Blocking H1 receptors in the hypothalamus increases appetite and carbohydrate cravings
• 5-HT2C receptor antagonism: this further contributes to appetite stimulation
• Improved mood and sleep: as depression lifts, appetite may naturally recover, compounding the pharmacological effect

How much weight gain to expect:

Studies suggest an average gain of 2 to 4 kg over the first 6 to 12 months. However, some patients gain considerably more, particularly those who were underweight at baseline due to depression.

Management strategies:

• Monitor weight from the start of treatment
• Aim for a balanced, portion-controlled diet
• Be aware of increased cravings, especially for sugary and starchy foods, and plan healthier alternatives
• Regular physical activity is strongly recommended
• If weight gain is severe and causing distress, discuss alternative antidepressants with your prescriber

NICE notes that patient preference, including concerns about weight gain, should guide the choice of antidepressant. Alternatives such as sertraline or fluoxetine are less likely to cause weight gain.

One of the most distinctive features of mirtazapine is its unusual relationship between dose and sedation.

Lower doses are more sedating:

At 15 mg (the usual starting dose), mirtazapine's antihistamine (H1) activity dominates. This produces marked drowsiness, which is why many prescribers recommend taking it at bedtime.

Higher doses may be less sedating:

At doses of 30 mg and above, increased noradrenergic (alpha-2 antagonist) activity begins to counterbalance the antihistamine sedation.

Many patients report feeling less drowsy when the dose is increased, which may seem counterintuitive.

Clinical implications:

• If a patient is troubled by persistent daytime drowsiness at 15 mg, increasing the dose to 30 mg may actually reduce sedation while improving antidepressant efficacy
• Conversely, reducing the dose from 30 mg to 15 mg does not necessarily reduce side effects and may worsen drowsiness
• The timing of the dose matters: taking mirtazapine 1 to 2 hours before bedtime can turn the sedation into a therapeutic advantage for patients with insomnia

This paradox is important for prescribers and patients to understand. If you are struggling with tiredness on mirtazapine, a dose increase rather than a decrease may be the answer.

Always discuss any dose changes with your doctor.

While mirtazapine is generally well tolerated, some serious side effects require prompt medical attention.

Blood disorders (rare):

Mirtazapine can, in rare cases, cause a reduction in white blood cells (agranulocytosis or neutropenia). This typically occurs within the first few weeks of treatment.

• Unexplained fever or sore throat
• Mouth ulcers
• Signs of infection that do not resolve

The BNF recommends that patients are advised to report these symptoms promptly.

Serotonin syndrome:

This is a rare but potentially life-threatening condition that can occur when mirtazapine is combined with other serotonergic medicines (e.g. SSRIs, tramadol, triptans).

Symptoms include agitation, tremor, high temperature, rapid heart rate and muscle rigidity. Seek emergency help immediately.

Suicidal thoughts:

As with all antidepressants, there is a small increased risk of suicidal ideation, particularly in younger adults (under 25) and during the first few weeks of treatment.

The MHRA advises close monitoring during this period.

Hyponatraemia:

Low sodium levels can occur, particularly in elderly patients. Symptoms include confusion, drowsiness and muscle cramps.

Seizures:

Rarely, mirtazapine may lower the seizure threshold. Use with caution in patients with a history of epilepsy.

Contact your GP, call NHS 111 or attend A&E if you experience any of these serious effects.

Most mirtazapine side effects are manageable with simple strategies. Here are evidence-based approaches for the most common issues.

For drowsiness:

• Take your dose 1 to 2 hours before bedtime
• Discuss a dose increase with your prescriber if daytime sedation is problematic (the dose-sedation paradox)
• Avoid alcohol, which worsens sedation
• Allow 2 to 3 weeks for tolerance to develop

For weight gain and increased appetite:

• Plan meals in advance to avoid impulsive eating
• Keep healthy snacks available for when cravings strike
• Regular weighing (weekly) helps identify trends early
• Exercise regularly, aiming for at least 150 minutes of moderate activity per week

For dry mouth:

• Sip water frequently throughout the day
• Sugar-free chewing gum or lozenges can stimulate saliva
• Good oral hygiene is important to prevent dental problems

For constipation:

• Increase fibre intake gradually
• Drink plenty of fluids
• Stay physically active

For vivid dreams:

• These usually settle within the first few weeks
• Avoiding caffeine and screens before bed may help

If side effects are persistent and affecting your quality of life, speak to your prescriber.

Alternative antidepressants are available, and the choice should be made collaboratively with your doctor based on NICE guidelines for depression.