Weight Gain, Drowsiness, and More: 10 Mirtazapine Side Effects to Know

If you’re starting or considering mirtazapine (Remeron), you probably have questions about possible side effects. Mirtazapine is FDA approved to treat depression, and it’s also prescribed off-label for anxiety and sleep. But like any medication, it can cause unwanted side effects.

Here’s a straightforward look at common, rare, and serious mirtazapine side effects you might experience, and what you can do about them.

Mirtazapine side effects at a glance

Many mirtazapine side effects are mild and can be managed at home. And oftentimes, they improve as your body gets used to the medication. While less common, more serious side effects are also possible.

Common mirtazapine side effects (reported by at least 2% of people during clinical trials) include:

• Drowsiness

• Increased appetite

• Weight gain

• Dizziness

• Nausea

• Weakness

• Dry mouth

• Back or muscle pain

• Flu-like symptoms

• Swelling in your limbs

• Abnormal dreams

• Tremor

• Confusion

• Peeing more often

• Higher cholesterol and triglyceride levels

Rare but serious mirtazapine side effects include:

• Boxed warning for suicidal thoughts

• Serotonin syndrome

• Low white blood cell counts

• Angle-closure glaucoma

• QT prolongation

• Drug reaction with eosinophilia and systemic symptoms (DRESS)

• Low sodium levels

• Liver test changes

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Serious side effects such as QT prolongation and serotonin syndrome may be more likely if you take mirtazapine with other medications that have similar risks. Your healthcare team can review your current medication list to identify these potential interactions before they become an issue.

Below, we look closer at 10 mirtazapine side effects and how to manage them.

1. Weight gain

Mirtazapine is more likely to cause weight gain than other antidepressants. That’s because it can increase your appetite, making you feel hungrier than normal. To manage potential weight gain, incorporate a nutritious diet and regular exercise into your routine. Adopting these habits can help with your depression symptoms, too.

If you notice weight gain, talk to your healthcare team to determine whether it's caused by mirtazapine. This may be one of the effects that stabilizes over time as you get used to the medication. But if weight gain is a concern for you, there are other antidepressant options — such as fluoxetine (Prozac) — which are less likely to cause weight gain.

2. Drowsiness

Drowsiness (feeling sleepy) is a common mirtazapine side effect — with more than 50% of people in clinical trials reporting it. It’s recommended that you take mirtazapine at bedtime to best manage this side effect.

Drowsiness from mirtazapine may be beneficial for people with insomnia. But it can be bothersome for others. After taking mirtazapine, be sure to avoid driving or doing other tasks that may be dangerous if you’re drowsy. It’s also a good idea to avoid alcohol, since it can make this side effect worse.

EXPERT PICKS: WHAT TO READ NEXT

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• A carefully calculated prescription: Understand which mirtazapine dosages work best for depression treatment.

If your drowsiness doesn’t go away, or if it becomes worse, talk to your prescriber. They can help decide what action to take.

3. Dizziness

Dizziness is another common side effect of mirtazapine. If you feel dizzy after taking mirtazapine, move slowly when sitting up or standing. This can help you avoid falls, which can be dangerous — especially for older adults. 

Tell your prescriber if your dizziness worsens or doesn’t go away over time. Dizziness can be caused by things other than medication. Your prescriber can help figure out the cause and the best way to manage it.

4. Dry mouth

Mirtazapine can cause dry mouth. About 1 in 4 people experienced dry mouth in clinical studies. Drinking plenty of water can help relieve this side effect. You can also try sugar-free gum or candy as well as over-the-counter (OTC) saliva substitutes.

5. Constipation

Constipation is another possible side effect of mirtazapine. It’s related to the “drying out” effect of mirtazapine that also causes dry mouth. To address constipation, be sure to drink plenty of water. If that doesn’t work, you can also try an OTC stool softener or laxative.

Constipation can be caused by many factors, so talk to your prescriber if it becomes a problem for you.

6. Nausea

Mirtazapine can cause nausea, but it’s less common than other side effects discussed above. Compared to other antidepressants, such as sertraline (Zoloft), nausea may be less likely with mirtazapine.

If you experience nausea, try taking mirtazapine with food. You can also try eating foods, teas, or supplements that contain ginger. If nausea doesn’t improve with time, or after you’ve made changes to how you take mirtazapine, let your prescriber know.

7. Low sodium levels

Mirtazapine can sometimes cause low sodium levels. This is a rare side effect of mirtazapine, but it’s also a serious one. Symptoms of low sodium levels include:

• Headache

• Confusion

• Weakness

• Difficulty with concentration and memory

If you experience any of these, talk to your healthcare team right away. More serious symptoms include hallucinations, fainting, or seizures. Seek emergency care right away if you experience these symptoms.

8. Higher cholesterol levels

Mirtazapine can sometimes cause cholesterol and triglyceride levels to rise, which may be linked to weight gain from the medication.

Your healthcare team will likely monitor your cholesterol levels regularly while you’re taking mirtazapine. Lifestyle changes, such as adjusting your diet and increasing physical activity, can also help manage these effects.

9. Flu-like symptoms

Experiencing flu-like symptoms while taking mirtazapine may not seem like a big deal. But it can actually be a sign of a rare but serious side effect. Flu-like symptoms could be a sign of a low number of white blood cells in your body. White blood cells help you fight infections. If your levels fall too low, you can feel like you have the flu.

Symptoms to watch out for include:

• Fever

• Chills

• Sore throat

• Mouth and nose sores

If you experience these symptoms, inform your healthcare team right away.

10. Suicidal thoughts

Mirtazapine has a boxed warning (the FDA’s strongest medication warning) about an increased risk of suicidal thoughts and behaviors in adolescents and young adults under 25 years old. 

All antidepressants have this warning due to findings of higher risk in this age group.

If you notice any changes in mood or behavior, especially when starting mirtazapine or adjusting your dose, tell your healthcare team right away.

If you or someone you know is having thoughts of suicide, help is available. Call 988 to reach the National Suicide Prevention Lifeline or text HOME to 741-741 to reach the Crisis Text Line.

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Written by:

Hindu Rao, PharmD, APh, BCACP

Hindu Rao, PharmD, APh, BCACP is a clinical assistant professor of pharmacy practice at Chapman University School of Pharmacy and a clinical pharmacist at Providence Medical Foundation. She specializes in chronic disease management and has her board certification in ambulatory care.

Edited by:

Alyssa Billingsley, PharmD

Alyssa Billingsley, PharmD, is the director of pharmacy content for GoodRx. She has over a decade of experience as a pharmacist and has worked in clinical, academic, and administrative roles.

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