Mirtazapine is a generic prescription drug that’s used to treat major depressive disorder in adults. It’s also available as the brand-name drug Remeron. Mirtazapine can cause side effects, including weight gain and dizziness.

Keep reading to learn about the common, mild, and serious side effects that mirtazapine can cause. For a general overview of the drug, including details about its uses, see this article.

Mirtazapine has a boxed warning. A boxed warning is the most serious warning from the Food and Drug Administration (FDA). For details, see the “Side effects explained” section.

Some people may experience mild to serious side effects during their mirtazapine treatment. Examples of the drug’s commonly reported side effects include:

  • extreme drowsiness
  • increased appetite
  • unintentional weight gain
  • dizziness

Mild side effects have been reported with mirtazapine. These include:

In most cases, these side effects should be temporary. And some may be easily managed. But if you have symptoms that are ongoing or bothersome, talk with your doctor or pharmacist. And do not stop taking mirtazapine unless your doctor recommends it.

Mirtazapine may cause mild side effects other than those listed above. See the drug’s prescribing information for details.

* To learn more about this side effect, see the “Mirtazapine’s side effects explained” section.

Serious side effects have been reported with mirtazapine. These include:

If you develop serious side effects while taking mirtazapine, call your doctor right away. If the side effects seem life threatening or you think you’re having a medical emergency, immediately call 911 or your local emergency number.

* To learn more about this side effect, see the “Mirtazapine’s side effects explained” section.

Note: After the FDA approves a drug, it tracks and reviews side effects of the medication. If you’d like to notify the FDA about a side effect you’ve had with mirtazapine, visit MedWatch.

ALLERGIC REACTION

For some people, Mirtazapine can cause an allergic reaction.

But it’s not clear how often this side effect occurred in the drug’s studies.

In general, symptoms of allergic reactions can be mild or serious.

What might help

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. They may suggest a treatment to manage your symptoms and can determine whether you should keep taking mirtazapine.

If you have symptoms of a severe allergic reaction, such as swelling or trouble breathing, call 911 or your local emergency number right away. These symptoms could be life threatening and require immediate medical care.

If your doctor confirms you’ve had a serious allergic reaction to mirtazapine, they may have you switch to a different treatment.

Get answers to some frequently asked questions about mirtazapine’s side effects.

Are older adults more likely to experience mirtazapine’s side effects?

Based on clinical studies, older adults may be at higher risk of experiencing side effects compared to younger adults taking mirtazapine. This is because mirtazapine is broken down and removed from the body slower in older adults. This allows the drug to stay in the body longer, leading to possible side effects.

Due to this risk, doctors will typically prescribe the lowest mirtazapine dosage in older adults with adjustments over time. Ultimately, they will prescribe the smallest dosage that provides the desired effect.

Talk with your doctor if you have questions about what to expect with mirtazapine treatment.

What are the long-term side effects of mirtazapine?

Long-term side effects include those that may start at any time you’re taking a drug, even if you’ve taken it for a long time. It also includes side effects that may not go away, even after you stop taking the drug.

Examples of long-term side effects reported in mirtazapine’s studies include:

  • severe skin reactions, including DRESS
  • increased cholesterol and triglycerides

Talk with your doctor to learn more about possible side effects of mirtazapine and how long they may last

Are there any side effects that only occur in females when taking mirtazapine?

Most often not. The majority of mirtazapine’s side effects can occur in both females and males.* However, in females, it takes longer for the body to fully get rid of mirtazapine. This means that it’s possible for females to experience more side effects than males.

Talk with your doctor if you have questions about what to expect with mirtazapine treatment.

* In this article, we use the terms “male” and “female” to refer to someone’s sex assigned at birth. For information about the difference between sex and gender, see this article.

Learn more about some of the side effects mirtazapine may cause.

Suicidal thoughts and behaviors

Mirtazapine has a boxed warning for suicidal thoughts and behaviors. A boxed warning is the most serious warning from the FDA.

Studies have shown that this side effect is more likely to occur in people ages 24 years and younger. For this reason, mirtazapine is not approved for use in children. People taking antidepressants more commonly experience this side effect during the first few months of treatment and during dosage changes. It’s more often reported in people taking antidepressants to treat major depression disorder. How long this side effect lasts is unclear.

Symptoms of suicidal thoughts and behaviors may include:

  • attempts to harm oneself
  • aggressive, angry or violent behavior
  • new or worsening depression or anxiety
  • extreme increase in energy, activity, or talking (mania)
  • trouble sleeping
  • other unusual changes in mood or behavior

What might help

Your doctor will watch you for symptoms of suicidal thoughts and behaviors while you’re taking mirtazapine. Be sure that your close family and friends and are also aware of these signs and symptoms. Talk with your doctor right away if any of these symptoms occur with you. If these symptoms are life threatening, call 911 or your local emergency number immediately. You can also call or text the 988 Suicide & Crisis Lifeline at 988.

Make sure you talk with your doctor about your medical conditions and any medications you take. These include prescriptions, over-the-counter medications, and supplements. Your doctor will help you determine whether mirtazapine is the right medication for you.

Help is out there

If you or someone you know is in crisis and considering suicide or self-harm, please seek support:

  • Call or text the 988 Suicide and Crisis Lifeline at 988 or chat at 988lifeline.org. Caring counselors are available to listen and provide free and confidential support 24/7.
  • Text HOME to the Crisis Text Line at 741741 to connect with a volunteer crisis counselor for free and confidential support 24/7.
  • Not in the United States? Find a helpline in your country with Befrienders Worldwide.
  • Call 911 or your local emergency services number if you feel safe to do so.

If you’re calling on behalf of someone else, stay with them until help arrives. You may remove weapons or substances that can cause harm if you can do so safely.

If you’re not in the same household, stay on the phone with them until help arrives.

Sexual problems

Sexual problems were an uncommon side effect in studies of mirtazapine. Examples of sexual problems reported include increased sex drive and abnormal ejaculation.

What might help

Talk with your doctor right away if you’re experiencing any sexual problems. They may be able to recommend ways to help reduce this side effect. They can also prescribe medications to assist.

Keeping track of side effects

During your mirtazapine treatment, consider taking notes on any side effects you’re having. You can then share this information with your doctor. This is especially helpful when you first start a new drug or combination of treatments.

Your side effect notes can include things such as:

  • what dose of the drug you were taking when you had the side effect
  • how soon you had the side effect after starting that dose
  • what your symptoms were
  • how your symptoms affected your daily activities
  • what other medications you were taking
  • any other information you feel is important

Sharing notes with your doctor will help them learn more about how mirtazapine affects you. They can then use this information to adjust your treatment plan if needed.

Below is important information you should consider before starting treatment with mirtazapine.

Boxed warning: Risk of suicidal thoughts and behaviors

Mirtazapine has a boxed warning. A boxed warning is the most serious warning from the FDA. For details, see the “Side effects explained” section.

Warnings

Mirtazapine can sometimes cause harmful effects in people with certain conditions. This is known as a drug-condition interaction. Other factors may also affect whether mirtazapine is a good treatment option for you.

Talk with your doctor about your health history before you take mirtazapine. Be sure to tell them if any of the following factors apply to you:

Disclaimer: Healthline has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.