If you have breast cancer, your doctor might suggest Faslodex (fulvestrant) as a treatment option for you. Along with other questions you have about the drug, you may be wondering about its side effects.

Faslodex is a prescription drug. It’s used to treat certain forms of advanced or metastatic breast cancer* that are hormone receptor-positive (HR-positive) in females.† Your doctor may recommend Faslodex if you’ve gone through menopause or if you’ve already tried a similar type of drug that didn’t work for you.

Depending on your breast cancer, you may use Faslodex with certain other cancer drugs.

Faslodex comes as a liquid medication for injection. A healthcare professional will use two syringes to give you two injections, one into the muscle of each buttock.

For more information about Faslodex, including details about its uses, see this in-depth article on the drug.

Like other drugs, Faslodex can cause mild or serious side effects. Knowing the possible side effects of Faslodex can help you and your doctor decide whether to add it to your treatment plan. Keep reading to learn more.

* Advanced breast cancer is cancer that has spread to areas near the breast or to lymph nodes in the nearby armpit. Metastatic breast cancer is cancer that has spread from the breast to other parts of the body.
† In this article, we use the term “female” to refer to someone’s sex assigned at birth. For information about the difference between sex and gender, see this article.

Some people may experience mild or serious side effects during their Faslodex treatment. Examples of more commonly reported side effects of Faslodex include:

  • injection site pain*
  • nausea
  • bone pain
  • back pain
  • increased levels of liver enzymes in your blood*

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

These side effects can vary depending on whether Faslodex is used along with other drugs, such as Kisqali (ribociclib), Ibrance (palbociclib), or Verzenio (abemaciclib).

Faslodex can cause mild side effects in some people. Examples of mild side effects that have been reported with Faslodex include:

* To learn more about this side effect, see “Side effects explained” 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 using Faslodex unless your doctor recommends it.

Faslodex may cause mild side effects other than the ones listed above. See the Faslodex patient 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 Faslodex, visit MedWatch.

Less commonly, Faslodex can cause serious side effects in some people. Serious side effects that have been reported with Faslodex include:

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

If you develop serious side effects while taking Faslodex, 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 Faslodex’s side effects.

How do the side effects of Ibrance compare with those of Faslodex?

Faslodex and Ibrance can cause some similar side effects. Examples include fatigue (lack of energy), nausea, vomiting, and feeling weak.

The two drugs can also cause different side effects. Some of the differences are due to the drugs being available in different forms. Ibrance comes as a tablet that you swallow. So it doesn’t cause the injection-related side effects that are common with Faslodex.

The following side effects occur more often with Ibrance than with Faslodex:

If you have questions about the side effects of Ibrance and Faslodex, talk with your doctor or pharmacist.

Can Faslodex cause weight gain?

No, Faslodex shouldn’t cause weight gain. This wasn’t reported as a side effect of the drug in studies.

It’s more likely that Faslodex would cause weight loss than weight gain. This is because reduced appetite can be a side effect of the drug in some people.

Weight loss has been reported in some people who received Faslodex with Verzenio (abemaciclib).

What are some of the long-term side effects of Faslodex?

Faslodex is meant to be used as a long-term treatment. Although not common, some side effects may be long term.

For example, Faslodex injections have been reported to cause damage to the sciatic nerve. This nerve is located near your buttocks. The muscle of your buttocks is where a healthcare professional injects the drug.

If this nerve is damaged, you could develop long-term sciatica-like symptoms. These include pain, numbness, and burning sensations that travel down the back of your leg.

If you have concerns about possible long-term side effects from Faslodex, talk with your doctor.

How long do Faslodex side effects last?

For most people, Faslodex’s side effects are mild and short term. Some side effects, such as injection site pain, typically reappear after you receive each dose. But these should ease a few days after your injections.

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


Fatigue (lack of energy) can be a common side effect of Faslodex.

When people used Faslodex by itself, fatigue was reported slightly more frequently with 500 milligrams (mg) of the drug than with 250 mg.

Fatigue is usually a mild side effect, but severe fatigue has been reported. In studies of Faslodex use with Ibrance (palbociclib) or Verzenio (abemaciclib), some people developed fatigue so severe that they stopped using these drugs.

What might help

If you’ve developed fatigue from Faslodex, it may improve on its own within a few days or weeks after a dose. If it doesn’t, here are a few tips that might help to ease your fatigue:

  • Make sure you get plenty of sleep. Aim for at least 7 to 8 hours of sleep each night. If you feel tired during the day, try taking a short nap.
  • Eat a balanced diet. Even if you don’t have much appetite, choose foods that are nutritious instead of foods with empty calories. For example, reach for veggies or fruits instead of crackers or chips. Nutrient-rich foods can help give you more energy. If you’re not sure what to eat, talk with your doctor or a dietitian.
  • Get regular physical activity. You might think that exercise would make you feel more tired. But studies show that increased physical activity significantly reduces fatigue in people with breast cancer. If you can, go for a daily walk, or try another mild form of exercise. This may provide a boost of energy. Talk with your doctor about what types of exercise might be appropriate for you.

It’s important to note that breast cancer itself can cause fatigue. This is especially true for advanced and metastatic breast cancer.* While these tips may help to ease fatigue, they may not make it go away completely.

Overall, Faslodex is an effective medication that may help to treat your cancer. While Faslodex can cause fatigue, the benefits of the treatment usually outweigh the risk of this side effect.

If you develop fatigue during Faslodex treatment, talk with your doctor. They may have other recommendations for treating this side effect.

* Advanced breast cancer is cancer that has spread to areas near the breast or to lymph nodes in the nearby armpit. Metastatic breast cancer is cancer that has spread from the breast to other parts of the body.

Hair loss

Although rare, hair loss was reported as a side effect in people using Faslodex on its own.

When Faslodex is used with certain drugs to treat breast cancer, hair loss is a common side effect. Other drugs commonly prescribed with Faslodex include Kisqali (ribociclib), Ibrance (palbociclib), and Verzenio (abemaciclib).

What might help

Hair loss is a common side effect of many cancer treatments. The good news is that hair loss with most cancer treatments usually isn’t permanent. After you complete your treatment, your hair should grow back.

During Faslodex treatment, treating your hair gently may help reduce hair loss. This includes not brushing it too much or using styling tools that are harsh on your hair, such as a blow-dryer or curling iron. Avoid washing your hair every day. When you do, use a gentle, fragrance-free shampoo.

If you have questions about hair loss with Faslodex, talk with your doctor.

Injection site pain or nerve damage

Unless you have liver problems, you’ll likely receive Faslodex as two injections, one in the muscle of each buttock. You’ll typically have these injections once a month.

As with any injected medication, some discomfort during a Faslodex injection is common. You’ll feel a pinch as you get your injection. Mild pain or swelling around the injection area may last for a few days after your dose.

Less commonly, sciatic nerve damage has been reported with Faslodex treatment. This nerve is located near your buttocks.

If this nerve is damaged, it’s possible that you could develop long-term sciatica-like symptoms. These symptoms include:

  • pain
  • numbness
  • burning sensations that travel down the back of your leg

What might help

To help ease pain, your doctor may apply a numbing cream, such as lidocaine, on the area before your injection. It may also help to try relaxing your muscles as much as possible during the injection.

If your injection site pain doesn’t go away or becomes severe, talk with your doctor. Also let them know if you develop numbness or pain that shoots down the back of your leg. Your doctor may recommend other ways to ease this side effect.

Increased liver enzymes in your blood

Faslodex may cause the levels of liver enzymes in your blood to become elevated. This was reported as a common side effect. It can happen because the body metabolizes (breaks down) Faslodex in the liver.

Slightly elevated liver enzymes aren’t usually a concern. But large increases in your liver enzyme levels can sometimes be a sign that your liver is damaged. If this occurs, you may also develop symptoms of liver damage, such as:

What might help

You might not have any symptoms of increased liver enzymes in your blood. Your doctor may check for this with a simple blood test called a liver function test.

Depending on your test results, your doctor may recommend lowering your dose of Faslodex. If your liver damage is serious, they’ll likely stop treatment with this drug.

If you’ve had any liver problems in the past, such as hepatitis, it’s important to let your doctor know before you start using Faslodex.

If you have any questions about increased liver enzymes with Faslodex, talk with your doctor.

Allergic reaction

Like most drugs, Faslodex can cause an allergic reaction in some people.

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. They may suggest an over-the-counter antihistamine you take by mouth, such as Benadryl (diphenhydramine), or a product you apply to your skin, such as hydrocortisone cream, to manage your symptoms.

If your doctor confirms you had a mild allergic reaction to Faslodex, they’ll decide if you should continue using 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 Faslodex, they may have you switch to a different treatment.

Keeping track of side effects

During your Faslodex 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 things such as:

  • what dose of drug you received from a healthcare professional 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 Faslodex affects you. And your doctor can use this information to adjust your treatment plan if needed.

Faslodex 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 take Faslodex. The list below includes factors to consider.

Bleeding disorders. Faslodex is given as two injections, one injection into the muscle of each buttock. This can cause bleeding and bruising at each injection site. If you already have a bleeding disorder, Faslodex injections could worsen your condition. Be sure to talk with your doctor about your bleeding disorder before starting Faslodex injections. They’ll help you decide whether this treatment is safe for you.

Blood thinners. If you’re taking a blood thinner medication, Faslodex injections can raise your risk for excessive bleeding. Be sure to tell your doctor and pharmacist about any prescription and over-the-counter drugs, supplements, and herbal remedies you take. They’ll guide you on the treatment plan that’s best for your condition.

Liver problems. Faslodex can cause increased liver enzymes in your blood. This can be a sign that the drug is damaging your liver. If you already have liver problems, such as hepatitis, taking Faslodex could worsen your condition. If your liver problem isn’t severe, your doctor will likely prescribe a lower dosage of Faslodex. But if it’s severe, your doctor will recommend a different treatment option that’s safer for you.

Allergic reaction. If you’ve had an allergic reaction to Faslodex or any of its ingredients, you should not take Faslodex. Ask your doctor what other medications are better for you.

Alcohol use and Faslodex

Alcohol isn’t known to cause any interactions with Faslodex.

But consuming too much alcohol can damage your liver. With liver damage or other liver problems, the usual dose of Faslodex may cause the drug to build up in your body.

If you drink alcohol, talk with your doctor about how much is safe to consume while you’re taking Faslodex.

Pregnancy and breastfeeding while taking Faslodex

You should not take Faslodex while you are pregnant or breastfeeding.

Using Faslodex during pregnancy may cause harmful effects to occur in a developing fetus.

Faslodex may also cause harm in a child who’s breastfed by a person using the drug. It’s recommended that you wait 1 year after your final dose of Faslodex before breastfeeding a child.

If you’re pregnant, breastfeeding, or have plans to become pregnant or breastfeed, talk with your doctor. They’ll guide you on the treatment plan that’s best for you at this time.

Most of Faslodex’s common side effects are mild, but serious side effects are possible. If you’re considering this drug as a treatment option, it’s important to talk with your doctor about any concerns you have.

Here are some questions to ask your doctor:

  • Do my other prescription drugs increase my risk for side effects with Faslodex?
  • Would any side effects of Faslodex be increased by taking certain over-the-counter medications?
  • Would a lower dosage decrease my chance of serious side effects from this drug?
  • Can I use natural remedies to help prevent or ease the side effects of Faslodex?

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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.