Firmagon (degarelix) is a prescription drug used to treat advanced prostate cancer in adults. Firmagon can cause side effects that range from mild to serious. Examples include injection site pain and hot flashes.

The active ingredient in Firmagon is degarelix. The drug comes as liquid solution that’s injected under the skin.

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

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

Mild side effects have been reported with Firmagon. 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. Do not stop Firmagon treatment unless your doctor recommends it.

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

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

If you develop serious side effects during Firmagon treatment, 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 “Side effects explained” section below.

Note: After the Food and Drug Administration (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 Firmagon, visit MedWatch.

Allergic reaction

For some people, Firmagon can cause an allergic reaction. This side effect didn’t occur in studies, but was reported since the drug was approved.

In general, symptoms of allergic reaction can be mild or serious. You can learn more about possible symptoms in this article.

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 continue Firmagon treatment.

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’ve had a serious allergic reaction to Firmagon, they may have you switch to a different treatment.

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

Does Firmagon cause long-term side effects?

It’s possible. 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’ve stopped treatment with the drug.

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

  • testicular atrophy (shrinking of the testes)*
  • abnormal heart rhythms, such as long QT syndrome*

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

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

How long do Firmagon’s side effects last?

The duration of Firmagon’s side effects can vary. Certain side effects, such as injection site reactions, may go away within a few days. Others can last throughout your treatment or continue even though you’ve stopped treatment. For example, testicular atrophy may not be reversible and could continue even after you’ve stopped Firmagon treatment.

Talk with your doctor or pharmacist if you experience any ongoing side effects.

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

Testicular atrophy

Testicular atrophy (shrinking of the testes) was a serious side effect reported in studies of Firmagon. Testicular atrophy may cause symptoms such as:

  • infertility
  • low sex drive
  • softer testes
  • reduced muscle mass
  • lessened facial or pubic hair growth

Factors that can increase the risk of testicular atrophy include age and consuming excessive amounts of alcohol. This is because both risk factors usually lead to low testosterone levels. If you have these risk factors during treatment with Firmagon, you’re more likely to experience testicular atrophy.

What might help

If you experience symptoms of testicular atrophy during Firmagon treatment, talk with your doctor right away. Be sure to tell them about all your medication conditions. Your doctor can help you decide whether Firmagon is the right treatment for you.

Abnormal heart rhythms

Abnormal heart rhythms, such as long QT syndrome, were reported in Firmagon studies but were rare. Long QT syndrome occurs when your heart’s electrical activity changes and is no longer normal. You may not experience any symptoms with long QT syndrome. If you do have symptoms, they may include:

  • dizziness
  • chest pain
  • fainting
  • a fluttering feeling in the chest
  • gasping noisily while sleeping

What might help

Talk with your doctor about all of your medical conditions before starting Firmagon. They may have you complete an electrocardiogram (EKG) before and during treatment. The EKG allows them to monitor your heart’s electrical activity.

Your doctor may also order tests to check your blood electrolytes before and during treatment. Talk with them right away if you notice any of the above symptoms during your Firmagon treatment. They may recommend stopping Firmagon and will prescribe a different treatment for your prostate cancer.

Keeping track of side effects

During Firmagon 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 Firmagon affects you. They can then use this information to adjust your treatment plan if needed.

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

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

  • heart problems, such as long QT syndrome
  • abnormal electrolyte levels
  • kidney or liver problems
  • previous allergic reaction to Firmagon
  • pregnancy
  • breastfeeding
  • alcohol consumption

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.