Enhertu (fam-trastuzumab deruxtecan-nxki) is a prescription drug that’s used to treat certain cancers, such as breast cancer. Enhertu can cause side effects that range from mild to serious. Examples include hair loss, nausea, and vomiting.

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

* The Food and Drug Administration (FDA) granted Enhertu accelerated approval for this use. A drug may receive accelerated approval if early studies suggest that 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.
† A tumor is a mass of cancerous tissue.

The active ingredient in Enhertu is fam-trastuzumab deruxtecan-nxki. (An active ingredient is what makes a drug work.) Enhertu is a biologic drug, which means the drug is made from parts of living organisms.

The drug comes as a powder that a healthcare professional mixes into a liquid to make a solution. They then give you the medication as an intravenous (IV) infusion (an injection into a vein that is given over time).

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

Listed below are just a few of the more common side effects reported by people who took Enhertu in studies. These side effects can vary depending on what condition the drug is used to treat.

More common side effects in people receiving Enhertu for breast cancer, non-small cell lung 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:

* To learn more about this side effect, see the “Side effects explained” section below.

Below are some of the mild side effects that may occur from receiving Enhertu. These side effects may vary depending on what condition the drug is given to treat.

Examples of mild side effects that have been reported in people receiving Enhertu for breast cancer, non-small cell lung cancer (NSCLC), or solid tumors include:

Examples of mild side effects that have been reported in people receiving Enhertu for stomach cancer or gastroesophageal cancer (cancer that forms in the area where the esophagus and stomach meet) include:

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

* To learn more about this side effect, see the “Side effects explained” section below.

In most cases, these side effects should be temporary. And some may be easily managed, too. But if you have any symptoms that are ongoing or that bother you, talk with your doctor or pharmacist. And don’t stop receiving Enhertu unless your doctor recommends it.

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

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks side effects of the medication. If you’d like to notify the FDA about a side effect you’ve had with Enhertu, visit MedWatch.

It’s possible to have serious side effects from Enhertu, though these are less likely to occur than mild side effects. Serious side effects that have been reported with Enhertu include:

* 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 “Side effects explained” section below.
† Enhertu has a boxed warning for risk of fetal harm. This is the most serious warning from the FDA. To learn more, see “Pregnancy and breastfeeding while using Enhertu” under the “Warnings for Enhertu” section below.
‡ To learn more about this side effect, see the “Side effects explained” section below.
§ An allergic reaction is possible after receiving Enhertu. But it’s not clear whether this side effect occurred in studies.

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

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

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. In some cases, side effects become so severe that Enhertu treatment needs to stop. This is referred to as “unacceptable toxicity.”

For example, if you develop lung problems* from Enhertu, your doctor may recommend pausing your treatment or decreasing your dose. If these 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.

* 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 “Side effects explained” section below.

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

During treatment with Enhertu, your doctor will monitor you for any side effects that may occur.

For instance, they’ll monitor your blood for low levels of red blood cells, white blood cells, or 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 dose decrease.

To lower your risk of serious side effects, it’s important that you have the tests that your doctor recommends. This can also help in finding side effects as soon as possible, so that they can be treated before they become serious.

You can monitor yourself for side effects during Enhertu treatment as well. 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 should be treated immediately.

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

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.*

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
  • provide you with nausea medications, steroids, or both prior to 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 decreasing your risk of serious side effects during Enhertu treatment, see the section just above.

* 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 “Side effects explained” section below.

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

Risk of serious lung problems

Enhertu has a boxed warning for the risk of serious lung problems. Boxed warnings are the most serious warnings from the Food and Drug Administration (FDA). They alert doctors and patients about drug effects that may be dangerous.

In rare cases, 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 it can be treated right away. Symptoms may include:

What might help

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. In some cases, they may recommend pausing your treatment or reducing your dose of Enhertu. Your doctor may also suggest taking a steroid medication, such as Rayos (prednisone), 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 make lung problems that already exist even worse. 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.

Hair loss

You may have hair loss during treatment with Enhertu. This was a common side effect reported in 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 might help

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 a few hours after. Your doctor can tell you more about options for scalp cooling.

For other tips that may help ease this side effect, see this article.

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 people receiving the drug.

What might help

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. In some cases, it’s best to eat bland foods.

For a few more nausea remedies to try, see this article.

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 might help

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 trouble 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 that 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 might help

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. To manage your symptoms, they may suggest an over-the-counter antihistamine you take by mouth, such as Benadryl (diphenhydramine). 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 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 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 to do 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.

Enhertu has several warnings that may affect whether you can safely receive this drug.

Boxed warnings

Enhertu has boxed warnings about the risk of serious lung problems and risk of fetal harm. Boxed warnings are the most serious warnings from the Food and Drug Administration (FDA).

  • Risk of serious lung problems: This medication can make lung problems that already exist even worse. If you have a lung 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. For details on serious lung problems, see the “Side effects explained” section above.
  • Risk of fetal harm: Receiving Enhertu during pregnancy can cause harm to a developing fetus. To learn more about fetal harm, see “Pregnancy and breastfeeding while using Enhertu” below.

Other warnings

Enhertu may not be right for you if you have certain medical conditions or other factors that affect your health. Talk with your doctor about your health history before you receive Enhertu. Below are some factors to consider.

Heart problems: Before starting Enhertu treatment, tell your doctor about any heart problems you have or have previously had. This medication can cause heart problems, which may make current heart conditions worse. If you have this kind of condition, your doctor may suggest frequent monitoring of your heart. Or they may recommend a different treatment option for you.

Allergic reaction: If you’ve had an allergic reaction to Enhertu or any of its ingredients, your doctor will likely not prescribe Enhertu. Ask your doctor what other medications are better options for you.

Active infection: Before you start receiving Enhertu, tell your doctor about any infections you currently have. This medication can affect your immune system, so it can decrease your body’s ability to fight infections. If you have an active infection, your doctor will likely recommend treating it before you start receiving Enhertu.

Alcohol and Enhertu

Alcohol isn’t known to interact with Enhertu, but it may make some of the side effects of the drug worse. For example, Enhertu may cause nausea, vomiting, and liver problems, and so can alcohol. As a result, drinking alcohol while receiving Enhertu may increase the risk or severity of these side effects.

If you drink alcohol and would like to do so during treatment with Enhertu, talk with your doctor about how much is safe for you to consume.

Pregnancy and breastfeeding while using Enhertu

If you’re pregnant or breastfeeding, or planning to become pregnant or breastfeed, your doctor will likely not recommend Enhertu to you.

Pregnancy and risk of fetal harm

Enhertu is not safe to receive during pregnancy. In fact, Enhertu has a boxed warning for the risk of fetal harm. Boxed warnings are the most serious warnings from the FDA. They alert doctors and patients about drug effects that may be dangerous.

Enhertu can cause harm to a developing 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.

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. For information about the difference between sex and gender, see this article.

Breastfeeding

It’s not known whether Enhertu passes into breast milk or whether it can affect a child who is breastfed. Due to the possible risk of harm to your child, you should not breastfeed while receiving Enhertu and for 7 months after completing treatment.

If you have any questions about pregnancy or breastfeeding during Enhertu treatment, talk with your doctor.

Before you start Enhertu treatment, be sure to discuss possible side effects of the medication with your doctor. Although most side effects of Enhertu are mild, it can cause severe side effects in rare cases. If you know what side effects may occur, you and your doctor may be able to treat them sooner.

Here are a few questions you may wish to ask your doctor:

  • How can I treat side effects that I have from receiving Enhertu?
  • Will my other medical conditions increase my risk of Enhertu’s side effects?
  • What should I do if I become pregnant during my treatment?

To learn more about Enhertu, see these articles:

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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. In some cases, 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 are still bothersome, 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.