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Faslodex: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on April 5, 2022.

1. How it works

  • Faslodex is a brand (trade) name for fulvestrant which may be used to treat certain types of breast cancer.
  • Faslodex (fulvestrant) works by binding to estrogen receptors in human breast cancer cells with as much affinity as estradiol. This downregulates the estrogen receptor protein, causing the receptor to break down, which prevents a normal response to estrogen. It also reduces the number of estrogen receptors in breast cancer cells. Many breast cancers have estrogen receptors and the growth of these tumors is stimulated by estrogen.
  • Faslodex belongs to the class of medicines known as estrogen receptor antagonists. It may also be called an antineoplastic hormone.

2. Upsides

  • May be used as monotherapy to treat hormone-receptor (HR)-positive advanced breast cancer in postmenopausal women with disease progression following endocrine therapy.
  • May also be used in postmenopausal women not previously treated with endocrine therapy to treat HR-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer.
  • In combination with ribociclib (Kisqali), Faslodex may be used to treat HR-positive, HER2-negative advanced or metastatic breast cancer in postmenopausal women as initial endocrine-based therapy.
  • In combination with ribociclib (Kisqali), palbociclib (Ibrance), or abemaciclib (Verzenio) may be used to treat HR-positive, HER2-negative advanced or metastatic breast cancer in women with disease progression following endocrine therapy.
  • Not considered a form of chemotherapy, although it is used in the treatment of breast cancer. Rather it is a type of endocrine or hormonal therapy that blocks estrogen receptors in the body that may fuel breast cancer. This helps slow the growth and spread of cancer. Hormone therapy may be prescribed after surgery to prevent the breast cancer from reoccurring, before surgery to help shrink the tumor size, or if the cancer returns later.
  • Given once monthly by intramuscular (into the muscle) injection into the buttocks (gluteal area), after an initial loading dose of three injections spaced apart on days 1, 15, 29 of the first month.
  • Research has shown Faslodex to be at least or more effective than Arimidex (anastrozole) after tamoxifen has stopped working.
  • Does not appear to be associated with an increased risk of endometrial cancer, unlike tamoxifen.
  • Does not appear to have unfavorable effects on lipid profiles, unlike letrozole and exemestane.
  • No dosage adjustment is needed for kidney disease.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Injection site pain, headache, back pain, fatigue, asthenia (feeling of weakness), hot flashes, joint or bone pain, nausea, vomiting, weight loss, constipation, muscle pain, cough, and shortness of breath are the most common side effects reported.
  • Injection site reactions have included sciatica, neuralgia, neuropathic pain, and peripheral neuropathy. Be cautious when administering Faslodex at the dorsogluteal injection site due to the underlying proximity of the sciatic nerve.
  • Other reported side effects include fluid retention, sore throat, sweating, rash, and urinary tract infections.
  • Faslodex is usually given as two 5mL injections of 250mg Faslodex, one in each buttock (to make a total dose of 500mg). Each should be injected slowly over one to two minutes.
  • May increase the risk of bleeding and should be used with caution in people with thrombocytopenia, taking anticoagulants, or with a pre-existing bleeding tendency.
  • Not effective for hormone-receptor-negative breast cancer.
  • May interfere with laboratory tests for estradiol causing falsely elevated estradiol levels.
  • Adjust dosage in moderate liver disease to 250mg. Faslodex has not been studied in women with severe liver disease.
  • May not be suitable for some people including those with severe liver disease or with thrombocytopenia. Avoid in people who are hypersensitive to Faslodex.
  • May cause nerve damage related to the injection. Call your doctor if you develop any of the following symptoms in your legs following injection: numbness, tingling, or weakness.
  • Requires administration by a health professional.
  • Can cause fetal harm when administered to a pregnant woman. Advise women to use effective contraception and not to breastfeed while undergoing treatment with Faslodex and for one year following the last dose. May impair fertility.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

Faslodex may be used alone or in combination with other agents for the treatments of certain types of breast cancer in postmenopausal women or women with disease progression following endocrine therapy. Faslodex is given by injection into the muscles of the buttock once a month by a healthcare provider. Injection site reactions are the most common side effect and it is at least as effective as anastrozole.

5. Tips

  • Keep your appointments with your healthcare provider for your Faslodex injection. Faslodex is administered into the muscle of each buttock.
  • Report any injection site reactions, such as numbness, tingling, or weakness, allergic-type reactions, or any other worrying side effects, such as shortness of breath, pain, or cough to your doctor.
  • Tell your doctor if you have any bleeding disorders or are taking any medications or supplements that may increase the risk of bleeding such as warfarin. If you experience bleeding after your injection, tell your doctor.
  • Ensure you use adequate contraception while you are taking Faslodex so you don't become pregnant as Faslodex can harm an unborn baby while you are being administered it and for up to one year after the last dose. If you inadvertently become pregnant tell your doctor right away. Do not breastfeed while being administered Faslodex and for one year after the last dose.

6. Response and effectiveness

  • Once Faslodex is given by intramuscular (into the muscle) injection it takes 7 days to reach peak levels. Even though Faslodex has reached peak plasma levels and is blocking and reducing the number of estrogen receptors, the effect of Faslodex on your breast cancer will not be noticeable for some time.
  • Faslodex is given as an injection every two weeks for the first three doses, then once a month thereafter. The half-life, which is how long it takes for half of the dose to be removed from the body, is approximately 40 days so the Faslodex remains effective for a long time. This means once you are on the monthly maintenance dose the Faslodex levels remain consistently at a therapeutic level.
  • There was no statistically significant difference in overall survival between Faslodex IM 250mg once monthly and anastrozole 1mg orally once a day, after an average follow-up of 28.2 months in one study and 24.4 months in another study.
  • The median progression-free survival was 16.6 months for 230 women administered Faslodex IM 500mg compared with 13.8 months for 232 women administered anastrozole 1mg orally daily.

7. Interactions

Medicines that interact with Faslodex may either decrease its effect, affect how long it works, increase side effects, or have less of an effect when taken with Faslodex. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

There are is currently only one reported drug interaction with Faslodex:

  • omacetaxine (may increase the risk of serious infections).

However, Faslodex has been associated with injection site reactions, including bleeding, and the risk of this may be higher in people already administered anticoagulants, such as warfarin or dabigatran.

Although Faslodex is metabolized by CYP3A4 hepatic enzymes in vitro, studies have shown no alteration of Faslodex pharmacokinetics when it has been administered with CYP3A4 inducers or inhibitors.

Please check the product information for Faslodex for any new interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use Faslodex only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2022 Revision date: April 5, 2022.