Enhertu (fam-trastuzumab deruxtecan-nxki) can cause mild to serious side effects. More common side effects include hair loss and nausea. If Enhertu side effects become bothersome, talk with your doctor about ways to manage them or other treatment options.

Enhertu is used in certain situations to treat the following kinds of cancer in adults. People receiving the drug must have received treatment for cancer in the past.

Keep reading to learn about the common, mild, and serious side effects Enhertu can cause.

Enhertu has boxed warnings. A boxed warning is the most serious warning from the Food and Drug Administration (FDA). For details, see the “Boxed warnings for Enhertu” section.

Note: The Food and Drug Administration (FDA) granted Enhertu accelerated approval for NSCLC and solid tumors. A drug may receive accelerated approval if early studies suggest it could be effective for certain conditions. Based on results from more studies, the FDA may decide to fully approve the drug or withdraw the approval.

Enhertu can cause certain side effects, some more common than others. Talk with your doctor if side effects do not go away, worsen, or become too difficult for you to tolerate.

The following lists contain a few of the more common side effects reported by people who took Enhertu in studies. These side effects can vary depending on the condition the drug is treating.

More common side effects in people receiving Enhertu for breast cancer, NSCLC, or solid tumors include:

More common side effects in people receiving Enhertu for stomach cancer or gastroesophageal cancer (cancer that forms in the area where the esophagus and stomach meet) include:

For more information about some of these side effects, see the “Side effect specifics” section.

Mild side effects that may occur from receiving Enhertu can vary depending on the condition you are receiving it to treat.

Mild side effects reported in studies of Enhertu for breast cancer, NSCLC, or solid tumors include:

Mild side effects reported in studies of Enhertu for stomach cancer or gastroesophageal cancer include:

  • fatigue
  • vomiting
  • constipation
  • diarrhea
  • decreased appetite
  • fever
  • nausea
  • hair loss

These side effects should mostly be temporary, and some may be easily manageable. But if you have any symptoms that are ongoing or bother you, talk with your doctor or pharmacist. Do not stop Enhertu treatment unless your doctor recommends it.

Enhertu may cause mild side effects other than the ones listed. See the Enhertu prescribing information for details.

For more information about some of these side effects and how to manage them, see the “Side effect specifics” section.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks and reviews side effects of the medication. Sharing your experience helps make medications safer for everyone by giving doctors and researchers more information about how the drug works in real life. If you develop a side effect while taking Enhertu and want to tell the FDA about it, visit MedWatch or call 800-332-1088.

Enhertu may cause serious side effects. The following list may not include all possible serious side effects of the drug. For more information, you can refer to Enhertu’s prescribing information.

If you develop serious side effects while taking Enhertu, 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.

Serious side effects that have been reported with Enhertu include:

For more information about some of these side effects, see the “Side effect specifics” and “Boxed warnings for Enhertu” sections.

Enhertu has boxed warnings. A boxed warning is the most serious warning from the FDA. They alert doctors and patients about drug effects that may be dangerous.

Risk of serious lung problems

Enhertu has a boxed warning about the risk of serious lung problems. Rarely, Enhertu can cause interstitial lung disease (ILD), which results in scarring or inflammation in the lungs. This can make it harder for you to breathe. It’s important to be aware of the symptoms of ILD so that you can receive treatment right away. Symptoms may include:

What you can do

Throughout Enhertu treatment, your doctor will monitor you for ILD by ordering blood work and other tests, such as a CT scan. This can help identify and manage ILD as soon as possible.

If you develop new or worsened symptoms of a lung problem, contact your doctor right away. Sometimes, they may recommend pausing your treatment or reducing your dose of Enhertu. Your doctor may also suggest taking a steroid medication, such as prednisone (Rayos), to reduce your symptoms.

Depending on the severity of your symptoms, your doctor may recommend that you stop receiving Enhertu and try a different treatment for your cancer.

This medication can worsen existing lung problems. If you have a condition that affects your breathing, such as asthma, be sure to tell your doctor before starting Enhertu. Your doctor may recommend a different treatment option for you.

If you have more questions or concerns about lung problems during Enhertu treatment, talk with your doctor.

Risk of fetal harm

Enhertu also has a boxed warning for the risk of fetal harm. Enhertu is not safe to receive during pregnancy.

Enhertu can cause harm to a fetus, including skeletal or lung development problems. It may even be life threatening to a newborn. Due to these risks, if you can become pregnant, your doctor will likely give you a pregnancy test before you start treatment.

What you can do

Females who can become pregnant should use birth control while receiving Enhertu and for 7 months after stopping treatment. Males with partners who can become pregnant should use birth control throughout treatment and for 4 months afterward.

If you become pregnant at any time during Enhertu treatment, tell your doctor right away. And if you have any questions about Enhertu and pregnancy, talk with your doctor.

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

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

Hair loss

You may have hair loss during treatment with Enhertu. This was a common side effect reported in studies of people receiving this medication. Hair loss can occur not only on your head but also on other areas of your body where hair grows. For example, it can affect your eyelashes, eyebrows, or hair on your legs and arms.

What you can do

To help with hair loss, avoid harsh products such as hairsprays or dyes and use a mild shampoo.

Scalp cooling may also help prevent hair loss. This type of treatment works by inhibiting blood flow to your scalp, which helps limit the amount of Enhertu that reaches the hair follicles there. Scalp cooling treatment starts before you receive an Enhertu infusion and continues for a few hours afterward. Your doctor can tell you more about options for scalp cooling.

If you’re concerned about hair loss while receiving Enhertu, talk with your doctor. They may have other suggestions for treating this side effect.

Nausea

You may have nausea from receiving Enhertu. This was the most common side effect reported in studies of the drug.

What you can do

If you develop nausea during Enhertu treatment, there are a few things you can try to reduce this side effect. Eating smaller meals more often may help decrease nausea. So instead of having three large meals each day, try eating six small meals or snacks.

In addition, consider avoiding foods that are very hot or cold or have strong smells. Sometimes, it’s best to eat bland foods.

If you’re still having nausea, your doctor may be able to prescribe a medication to help relieve this side effect. Talk with your doctor about the best ways to treat nausea while you’re receiving Enhertu.

Heart problems

Though rare, it’s possible to develop heart problems from receiving Enhertu. Heart problems from Enhertu may affect the left side of your heart. This, in turn, can affect your heart’s ability to pump blood throughout your body.

What you can do

Before you start receiving Enhertu, your doctor will likely test your heart to be sure it’s healthy enough for treatment. During treatment, your doctor will monitor you for symptoms of heart conditions, such as swelling of the arms or legs or difficulty breathing. In addition, your doctor may test your heart throughout treatment to monitor your heart function.

If your heart function worsens during treatment, your doctor may recommend pausing treatment, decreasing your dose, or stopping treatment altogether. If you have a history of heart disease or other heart conditions, your doctor may recommend a different treatment option for you.

If you have questions about the heart problems Enhertu may cause, talk with your doctor.

Allergic reaction

Like most drugs, Enhertu can cause an allergic reaction in some people. But it’s not clear whether this side effect occurred in studies.

Symptoms can be mild or serious and can include:

  • skin rash
  • itchiness
  • flushing (temporary warmth, redness, or deepening of skin color)
  • swelling under your skin, typically in your eyelids, lips, hands, or feet
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe

What you can do

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. They may suggest an over-the-counter antihistamine you take by mouth, such as diphenhydramine (Benadryl), to manage your symptoms. Or they may recommend a product you apply to your skin, such as hydrocortisone cream.

If your doctor confirms you had a mild allergic reaction to Enhertu, they’ll decide whether you should continue receiving it.

If you have symptoms of a severe allergic reaction, such as swelling or difficulty 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 had a serious allergic reaction to Enhertu, they may have you switch to a different treatment.

Keeping track of side effects

During Enhertu treatment, consider keeping notes on any side effects you’re having. Then, you can share this information with your doctor. This is especially helpful when you first start taking new drugs or using a combination of treatments.

Your side effect notes can include:

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

Keeping notes and sharing them with your doctor will help your doctor learn more about how Enhertu affects you. And your doctor can use this information to adjust your treatment plan if needed.

Here are answers to some frequently asked questions about Enhertu’s side effects.

How long do Enhertu side effects last?

It varies depending on the side effect as well as the person.

Mild side effects, such as nausea or diarrhea, may go away within a few days to a couple of weeks as your body gets used to Enhertu.

But some of Enhertu’s side effects may be long term or even permanent, though these tend to be rare. Examples include serious heart and lung problems. (Enhertu has a boxed warning for the risk of serious lung problems. For details, see the “Boxed warnings for Enhertu” section.)

Before you start Enhertu treatment, your doctor will determine your risk of serious side effects. This includes going over your health history and possibly having you get certain lab tests. To help prevent any side effects you may have a risk of, your doctor may:

  • decrease your dose
  • slow the rate of the infusion (you receive Enhertu as an intravenous infusion)
  • provide you with nausea medications, steroids, or both before starting Enhertu

If your risk of a serious Enhertu side effect is high, your doctor may recommend you try a different treatment for your cancer. To learn about lowering the risk of serious Enhertu side effects, see the “How can I decrease my risk of serious side effects from Enhertu?” section.

What is ‘unacceptable toxicity,’ and how may this affect Enhertu treatment?

Throughout treatment, your doctor will monitor you for the possibility of Enhertu toxicity (harm caused by Enhertu). Although many of Enhertu’s side effects are mild, the drug can cause some severe side effects. Sometimes, side effects become so severe that a person needs to stop Enhertu treatment. This is known as “unacceptable toxicity.”

For example, if you develop lung problems from Enhertu, your doctor may recommend pausing your treatment or decreasing your dose. (Enhertu has a boxed warning for the risk of serious lung problems. This is the most serious warning from the FDA. To learn more, see the “Boxed warnings for Enhertu” section.)

If these lung problems linger or worsen, this may be considered an unacceptable toxicity. In other words, Enhertu may be harmful to your health.

If you develop an unacceptable toxicity from Enhertu, your doctor will likely recommend that you stop Enhertu treatment. They may suggest a different treatment for your cancer.

How can I decrease my risk of serious side effects from Enhertu?

During treatment with Enhertu, your doctor will monitor you for any potential side effects.

For instance, they’ll monitor your blood for low levels of red blood cells, white blood cells, and platelets. Your doctor will check to make sure that these levels don’t become too low, which can increase your risk of infection. If your blood cell levels get too low during Enhertu treatment, your doctor may decrease your dose or pause your treatment.

In addition, Enhertu can cause heart problems. Before you start receiving this medication, you’ll have tests to see how well your heart is functioning. And you’ll have these tests throughout treatment to monitor your heart function. Depending on the results, your doctor may recommend a pause in Enhertu treatment or a decrease in the dose.

To lower your risk of serious side effects, it’s important to have the tests that your doctor recommends. This can help identify side effects as soon as possible so you can receive treatment before they become serious.

You can monitor yourself for side effects during Enhertu treatment. For example, if you develop a cough or fever after receiving Enhertu, tell your doctor right away. This may be a symptom of lung problems, which require immediate treatment.

For more ways to decrease your risk of serious side effects during Enhertu treatment, talk with your doctor.

Consider the following important information before starting Enhertu.

Enhertu has boxed warnings about the risk of serious lung problems and fetal harm. A boxed warning is the most serious warning from the FDA. For details, see the “Boxed warnings for Enhertu” section.

Other precautions

Before you receive Enhertu, discuss your health history with your doctor. Enhertu may not be right for you if you have certain medical conditions or other factors that affect your health. Be sure to talk with your doctor if any of the following apply to you:

  • heart problems
  • lung problems
  • allergic reaction to Enhertu or any of its ingredients
  • active infection
  • alcohol consumption
  • pregnancy
  • breastfeeding

Q:

Can my dose of Enhertu be changed if side effects become too bothersome for me?

Anonymous

A:

It’s possible that your doctor may change your dose of Enhertu based on any side effects you have from the drug. Sometimes, depending on how serious the side effect is, your doctor may recommend stopping Enhertu treatment.

For instance, if you develop interstitial lung disease and experience symptoms, your doctor will likely recommend that you permanently stop receiving Enhertu. They may suggest another drug for treating your cancer.

But if you have severely low levels of white blood cells or platelets, your doctor will likely pause your Enhertu treatment until these levels return to their usual state. Based on the condition you’re receiving Enhertu to treat, they may decrease your dose of Enhertu.

After two Enhertu dose decreases, if your side effects still bother you, your doctor will likely recommend that you permanently stop receiving the drug.

If you’re experiencing side effects from Enhertu, tell your doctor.

The Healthline Pharmacist TeamAnswers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.

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.