Anastrozole

Anastrozole is prescribed to treat:

• Hormone receptor-positive breast cancer in postmenopausal women • Various related conditions in the oncology category • Associated symptoms and complications

It is an important medication that helps manage these conditions effectively.

Anastrozole (Arimidex) is a non-steroidal aromatase inhibitor used in hormone receptor-positive breast cancer in postmenopausal women — as adjuvant therapy after surgery, for advanced disease, and for risk reduction in high-risk women. It works only after menopause, and understanding why explains most of how it is used.

Mechanism of Action

Aromatase converts androgens to estrogens. In premenopausal women the ovaries are the dominant estrogen source and produce it under pituitary control; after menopause, ovarian production ceases and the main source becomes peripheral aromatisation of adrenal androgens in adipose tissue, muscle, liver, and breast tissue itself.

Anastrozole competitively and reversibly inhibits aromatase, reducing circulating estrogen substantially in postmenopausal women. In hormone receptor-positive tumours, estrogen drives proliferation, so removing it removes the growth signal.

In premenopausal women the same inhibition triggers a compensatory response: falling estrogen releases the pituitary from negative feedback, gonadotropins rise, the ovaries are stimulated harder, and estrogen production increases rather than falls. Anastrozole is therefore ineffective — and potentially counterproductive — as monotherapy before menopause. Where an aromatase inhibitor is used in a premenopausal woman, it must be combined with ovarian suppression.

This is why menopausal status must be established rather than assumed, particularly in women whose periods stopped during chemotherapy, where ovarian function can recover.

Tamoxifen works differently — it blocks the estrogen receptor rather than reducing estrogen production — which is why it works in both premenopausal and postmenopausal women.

Clinical Use

In postmenopausal women with hormone receptor-positive early breast cancer, aromatase inhibitors reduce recurrence more effectively than tamoxifen and are standard adjuvant therapy, typically for five years and often extended.

Sequencing strategies are common: tamoxifen for a period followed by an aromatase inhibitor, or the reverse. In premenopausal women, tamoxifen alone or an aromatase inhibitor with ovarian suppression are both used depending on risk.

The three aromatase inhibitors — anastrozole, letrozole, and exemestane — are broadly similar in efficacy. Exemestane is steroidal and irreversible; the other two are non-steroidal and reversible. Switching between them is a reasonable step when side effects rather than efficacy are the problem, since tolerability differs between individuals more than effectiveness does.

Adherence is the central practical issue. These are drugs taken daily for years by women who feel well, to prevent a recurrence that may never come, and whose side effects are constant and unglamorous. Discontinuation rates are high, and non-adherence measurably increases recurrence. Treating the side effects seriously rather than encouraging patients to endure them is the intervention that keeps women on treatment. Our immunology team coordinates with oncology, and the American Cancer Society hormone therapy resource provides patient-level background.

Side Effects That Determine Whether Treatment Continues

Musculoskeletal symptoms are the dominant problem — joint pain and stiffness, particularly in the hands, wrists, and knees, often worst in the morning. They affect a large proportion of women and are the most common reason treatment is abandoned.

They are frequently under-treated because they are non-specific and easily attributed to age or to osteoarthritis. Measures that help include exercise — which is counterintuitive but genuinely effective — analgesia, vitamin D repletion, and switching to a different aromatase inhibitor, which resolves symptoms in a meaningful proportion.

Bone loss is the other major consideration. Estrogen deprivation accelerates bone turnover, and aromatase inhibitors cause measurable loss of bone density and increased fracture risk — unlike tamoxifen, which is bone-protective in postmenopausal women. Baseline bone density, calcium and vitamin D, weight-bearing exercise, and consideration of a bisphosphonate such as alendronate or zoledronic acid are part of routine care rather than optional additions. The bone health article covers the wider picture.

Vasomotor symptoms, vaginal dryness, and reduced libido are common. Vaginal estrogen is used cautiously and with oncology input given systemic absorption. Lipids may worsen modestly.

Monitoring and Follow-Up

Bone density is measured at baseline and at intervals during treatment. Calcium and vitamin D status should be assessed and repleted.

Lipids are checked periodically. Musculoskeletal symptoms should be asked about directly at every visit, since patients frequently do not volunteer them and simply stop the drug instead.

Adherence should be asked about non-judgementally and specifically — not "are you taking it" but "how many days in a typical week do you miss." Cost, side effects, and the psychological difficulty of a daily reminder of cancer are all real contributors and all addressable once identified.

Anastrozole has few significant drug interactions, which is an advantage. It should not be combined with estrogen-containing products, which would counteract it directly.

Special Populations

Anastrozole is used only in postmenopausal women, or in premenopausal women with concurrent ovarian suppression. Menopausal status should be confirmed biochemically where there is doubt, particularly after chemotherapy-induced amenorrhoea.

It is contraindicated in pregnancy and in premenopausal women without ovarian suppression. In women with existing osteoporosis or osteopenia, bone protection should begin alongside rather than after.

In men with breast cancer, aromatase inhibitors are used less often than tamoxifen, since the hormonal physiology differs. Hepatic impairment warrants caution; kidney impairment generally does not require adjustment.

Duration has extended as evidence has accumulated. Five years was standard for a long time; extended therapy beyond that reduces recurrence further in higher-risk disease, at the cost of continued side effects and bone loss. The decision balances recurrence risk against tolerability and bone health, and it is genuinely individual rather than protocol-driven — which means it is worth revisiting rather than defaulting to whatever was decided at the outset. The MedlinePlus anastrozole entry covers the prescribing detail.

When to Contact Your Doctor

Report joint pain and stiffness rather than enduring it — this is the most common reason women stop treatment, it is treatable, and switching agents often resolves it. Stopping silently is the outcome worth preventing.

Report a fracture from a minor fall, new back pain, or loss of height, which may indicate vertebral fracture. Report severe hot flushes affecting sleep or quality of life, which can be managed.

Report any vaginal bleeding, and any new breast lump or change. Report if cost or side effects are making it hard to take the medication consistently — that is a solvable problem and a far better conversation than an unreported gap in treatment.

To review side effects, discuss switching agents, or address bone health on hormone therapy, contact us or schedule a visit.