Anastrozole (Arimidex) for Breast Cancer: What Patients Should Know

By Sarah Friend, MD

Anastrozole, commonly known by the brand name Arimidex, is a medication frequently used to treat estrogen receptor-positive (ER+) breast cancer. It belongs to a class of medications called aromatase inhibitors and is a form of endocrine therapy, also commonly called hormone-blocking therapy.

For many people with ER+ breast cancer, anastrozole can play an important role in reducing the risk that breast cancer will return. It may also be used in several different settings—from ductal carcinoma in situ (DCIS, or Stage 0 breast cancer) to early-stage invasive breast cancer and metastatic disease.

Understanding why anastrozole is recommended, how it works, and what to expect from treatment can make this part of breast cancer care feel much less overwhelming.

How Does Anastrozole Work?

Many breast cancers use estrogen as a signal to grow. These cancers are called estrogen receptor-positive, or ER+.

Anastrozole works by blocking an enzyme called aromatase. Aromatase converts other hormones in the body into estrogen. By blocking this conversion, anastrozole dramatically lowers estrogen levels and reduces the estrogen available to stimulate ER+ breast cancer cells.

Importantly, anastrozole does not stop the ovaries from making estrogen. This is why aromatase inhibitors are primarily used in postmenopausal women, whose ovaries have already stopped producing significant amounts of estrogen due to menopause.

Aromatase inhibitors can also be used in premenopausal women, but the ovaries must first be suppressed. This is called ovarian function suppression and is typically accomplished with medications such as leuprolide (Lupron) or goserelin (Zoladex), which are given as injections to temporarily “turn off” ovarian estrogen production. An aromatase inhibitor such as anastrozole can then be used to block the remaining estrogen production elsewhere in the body.

Other commonly used aromatase inhibitors include letrozole (Femara) and exemestane (Aromasin).

Anastrozole for Early-Stage Invasive Breast Cancer

One of the most common uses of anastrozole is as adjuvant endocrine therapy for ER+ early-stage breast cancer.

“Adjuvant” means treatment given after surgery to reduce the risk that cancer will return.

Even after a breast cancer has been completely removed, microscopic cancer cells can sometimes remain elsewhere in the body. Endocrine therapy helps reduce the chance that ER+ breast cancer cells will grow in the future.

For postmenopausal women with ER+ breast cancer, an aromatase inhibitor such as anastrozole is one of the standard endocrine therapy options.

Some people take anastrozole from the beginning of endocrine therapy. Others may start with tamoxifen and later switch to anastrozole, or switch from anastrozole to another endocrine therapy because of side effects.

Treatment commonly lasts at least five years, although some people may benefit from a longer duration depending on their individual risk of recurrence and previous endocrine therapy.

Anastrozole for DCIS (Stage 0 Breast Cancer)

Anastrozole may also be used after treatment for estrogen receptor-positive ductal carcinoma in situ (DCIS), sometimes called Stage 0 breast cancer.

DCIS is made up of abnormal cancer cells that remain contained within the breast ducts and have not invaded the surrounding breast tissue.

After breast-conserving surgery for ER+ DCIS, endocrine therapy may be recommended to reduce the risk of developing another breast cancer event. This can include another area of DCIS or an invasive breast cancer in either breast.

In this setting, anastrozole is not being prescribed to improve survival, since survival after DCIS is already excellent. Instead, its purpose is to reduce the risk of a future breast cancer event.

For postmenopausal women with ER+ DCIS, anastrozole is one endocrine therapy option. Tamoxifen is another.

Can Anastrozole Be Given Before Surgery?

Yes. Although endocrine therapy is most commonly given after surgery, aromatase inhibitors such as anastrozole can sometimes be used before surgery.

Treatment given before surgery is called neoadjuvant therapy.

For selected postmenopausal patients with larger ER+/HER2-negative breast cancers, an aromatase inhibitor may be given for several months before surgery to shrink the tumor.

This approach may be useful as an alternative to chemotherapy when cytoreduction—shrinking the tumor before surgery—is recommended and the cancer is strongly hormone-sensitive.

Shrinking the tumor can sometimes make surgery easier or increase the possibility of breast-conserving surgery rather than mastectomy.

Anastrozole for Metastatic Breast Cancer

Anastrozole can also be used to treat ER+ metastatic breast cancer.

In metastatic disease, endocrine therapy is used differently than it is after surgery for early-stage breast cancer. The goal is to control the cancer for as long as possible while maintaining quality of life.

Anastrozole may be used alone in certain situations, although modern treatment for metastatic ER+/HER2-negative breast cancer frequently combines endocrine therapy with another targeted medication.

The best treatment depends on many factors, including previous treatments, tumor biology, genetic and molecular testing, where the cancer has spread, and how quickly it is progressing.

Anastrozole for Breast Cancer Risk Reduction

Anastrozole can also be considered for some postmenopausal women who have never had breast cancer but are at increased risk of developing it.

This may include women with certain high-risk breast findings or a calculated breast cancer risk high enough to consider preventive endocrine therapy.

In this setting, anastrozole is sometimes called “chemoprevention,” although it is important to understand that anastrozole is not chemotherapy. It is endocrine therapy designed to reduce estrogen stimulation of breast cells.

What Are the Most Common Side Effects?

Because anastrozole lowers estrogen levels, many of its side effects are related to estrogen deprivation.

Common side effects can include:

Joint pain or stiffness
Muscle aches
Hot flashes
Vaginal dryness
Changes in sexual health
Fatigue
Changes in cholesterol
Loss of bone density

Not everyone experiences these side effects, and their severity varies considerably from person to person.

Joint Pain and Stiffness

Joint and muscle symptoms are among the most frequently discussed side effects of aromatase inhibitors.

Some people notice stiffness when they first get out of bed or after sitting for a long period of time. Others develop discomfort in the hands, knees, hips, shoulders, or other joints.

Exercise and regular movement may help some patients. If symptoms become difficult to tolerate, it is important to discuss them with the oncology team rather than simply stopping treatment.

What If I Cannot Tolerate Anastrozole? The “Switching Strategy”

Side effects should not automatically mean that endocrine therapy has to be abandoned.

One helpful approach is what I call the “switching strategy.” If anastrozole causes difficult side effects, your oncologist may recommend switching to another aromatase inhibitor, such as letrozole (Femara) or exemestane (Aromasin).

Although all three aromatase inhibitors work in a similar way, an individual person may tolerate one significantly better than another. Someone who experiences troublesome joint pain, stiffness, or other symptoms on anastrozole may have fewer symptoms after switching to a different aromatase inhibitor.

Importantly, anastrozole, letrozole, and exemestane are considered to have equivalent efficacy for the adjuvant treatment of early-stage ER+ breast cancer. Switching from one aromatase inhibitor to another because of side effects does not mean that you are switching to a less effective treatment.

In appropriate circumstances, switching to tamoxifen may also be considered.

The important point is to communicate with your oncology team before stopping endocrine therapy altogether. If one medication is difficult to tolerate, there may be another option that is more manageable while still providing the important benefits of endocrine therapy.

Anastrozole and Bone Health

Estrogen helps protect bones, so lowering estrogen can gradually decrease bone mineral density and increase the risk of osteoporosis and fractures.

For this reason, bone health is an important part of treatment with anastrozole.

A baseline DEXA scan—a test that measures bone mineral density—is often obtained when starting an aromatase inhibitor and repeated periodically during treatment.

Depending on the individual, strategies to protect bone health may include:

Weight-bearing and resistance exercise
Adequate calcium intake
Maintaining an appropriate vitamin D level
Avoiding smoking
Limiting excessive alcohol intake
Monitoring bone density
Bone-strengthening medications when indicated

Your oncology team can help determine which measures are appropriate based on your individual bone density and fracture risk.

The Bottom Line

Anastrozole is one of the most commonly used medications for estrogen receptor-positive breast cancer.

It can be used after surgery to reduce the risk of recurrence, before surgery in selected patients to help shrink an ER+ tumor, after treatment for ER+ DCIS to reduce the risk of another breast cancer event, and as treatment for metastatic ER+ breast cancer. It can also be considered for breast cancer risk reduction in certain postmenopausal women at increased risk.

Like all cancer treatments, anastrozole has potential side effects—particularly joint symptoms and loss of bone density—but there are often strategies to help manage them.

If anastrozole is difficult to tolerate, talk with your oncology team before stopping it. Switching to another aromatase inhibitor may allow you to find a medication you tolerate better without sacrificing the efficacy of your endocrine therapy.

At MyFriendMD, our goal is to help you understand why a treatment has been recommended, what questions to ask, and how the different pieces of your cancer treatment plan fit together.

Medical Disclaimer

This article is intended for general educational and informational purposes only. It is not medical advice and should not be used to diagnose or treat any medical condition or to make decisions about starting, stopping, or changing cancer treatment. Treatment recommendations are individualized and may vary based on your diagnosis, medical history, and other factors. Always discuss questions about anastrozole, side effects, or your treatment plan with your oncologist or other licensed healthcare professional.

MyFriendMD provides educational and supportive cancer coaching and does not replace the care or medical advice of your treating healthcare team.

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