Arimidex (Anastrozole) 1 mg Tablets: Prescription Third-Generation Aromatase Inhibitor for Hormone Receptor-Positive Breast Cancer in Postmenopausal Women

Reviewed by the Lucas Clinic Medical Team | Updated May 2026

Arimidex (anastrozole) is a third-generation, non-steroidal aromatase inhibitor (AI) approved by the FDA in 1995. It is used in the treatment of hormone receptor-positive (HR+) breast cancer — the most common subtype of breast cancer, accounting for approximately 70–80% of all cases. Aromatase inhibitors work by blocking aromatase (CYP19A1), the enzyme responsible for converting androgens (primarily androstenedione and testosterone) into estrogens (estrone and estradiol) in peripheral tissues — adipose tissue, muscle, liver, and breast tissue. In postmenopausal women, who have no significant ovarian estrogen production, peripheral aromatisation is the primary source of circulating estrogen. Blocking aromatase in postmenopausal women reduces systemic estrogen levels by over 95%, depriving HR+ tumours of their primary growth signal.

Anastrozole is indicated for: (1) first-line treatment of locally advanced or metastatic HR+ breast cancer in postmenopausal women; (2) adjuvant treatment of early HR+ breast cancer in postmenopausal women (the definitive ATAC trial established superiority over tamoxifen for disease-free survival in this setting); and (3) adjuvant treatment of early HR+ breast cancer in premenopausal women after ovarian function suppression (in combination with LHRH agonist like goserelin). Anastrozole is prescription-only.

What Is Arimidex (Anastrozole)?

Anastrozole is a triazole derivative that competitively and reversibly inhibits the aromatase enzyme. Unlike steroidal aromatase inhibitors (exemestane) which form an irreversible suicide complex with aromatase, anastrozole is reversible — though clinical differences between reversible and irreversible AIs have not been established as pharmacologically meaningful in practice. Anastrozole achieves near-complete suppression of circulating estradiol to below 29 pmol/L (vs. baseline postmenopausal levels of 30–120 pmol/L). It is orally active, highly bioavailable (~85%), with a half-life of approximately 40–50 hours, making once-daily dosing appropriate. Steady-state plasma concentrations are reached within 7 days.

Anastrozole does not possess androgenic, progestogenic, estrogenic, or glucocorticoid activity at therapeutic doses. Unlike tamoxifen (which is a SERM with estrogenic agonist effects at some tissues), anastrozole is a pure estrogen-suppressing agent with no partial agonist activity. This distinction is clinically relevant — anastrozole has lower rates of endometrial cancer and thromboembolic events than tamoxifen (because it lacks tamoxifen's estrogenic effects on the endometrium and coagulation system) but is associated with greater loss of bone mineral density and worse musculoskeletal side effects (the arthralgias and joint pain that are a common reason for non-adherence).

Prescription vs. Over-the-Counter Status

Anastrozole is prescription-only in the US, UK, and all comparable markets. It is not available OTC under any approved formulation. It is prescribed and managed by oncologists, breast cancer specialists, and in some settings by GPs in ongoing stable adjuvant therapy phases. Off-label use by male bodybuilders and athletes to suppress estrogen during anabolic steroid cycles has generated an illicit market — this use is outside licensed indications, carries significant bone density risks, is not endorsed by any clinical guideline, and patients using anastrozole outside a medical context should be aware of the risks of profoundly low estrogen levels in males including fracture, dyslipidaemia, and psychological effects.

Available Strengths and Forms

Arimidex is available in a single oral strength of 1 mg film-coated tablets, taken once daily. This is both the starting and full therapeutic dose — dose escalation is not required or evidence-based. Generic anastrozole 1 mg tablets are widely available from multiple manufacturers and are bioequivalent to branded Arimidex. The drug is taken continuously for the adjuvant period — typically 5 years in standard protocols, sometimes up to 10 years in extended adjuvant protocols for high-risk early breast cancer (or sequential with tamoxifen, e.g., tamoxifen 2–3 years then anastrozole to complete 5 years of endocrine therapy). Can be taken with or without food. No dose adjustment is routinely needed for mild to moderate hepatic impairment; anastrozole is not recommended in severe hepatic impairment.

Price of Arimidex (Anastrozole)

Generic anastrozole 1 mg is one of the most affordable oncology drugs on the market since Arimidex's patent expired. A 30-tablet supply (one month's treatment) of generic anastrozole 1 mg costs approximately $15–$40 at US pharmacies without insurance, and often as low as $10–$15 with GoodRx discounts. Brand Arimidex (AstraZeneca) is significantly more expensive and rarely dispensed given affordable generic availability. In the UK, anastrozole is available on NHS prescription (standard prescription charge). Most insurance plans in the US cover generic anastrozole under their formulary at a low-tier co-pay. Financial assistance programmes exist for eligible uninsured patients through oncology-specific charities and manufacturer support programmes.

Frequently Asked Questions

What are the main side effects of anastrozole?

The most common and clinically significant side effects of anastrozole include: joint pain and stiffness (arthralgia/arthritis — affecting up to 35% of patients, the leading cause of non-adherence); hot flushes and menopausal symptoms (from profound estrogen suppression); bone mineral density loss and increased fracture risk (BMD screening and bisphosphonate co-therapy may be indicated); elevated cholesterol (lipid monitoring recommended); headache and fatigue; and vaginal dryness/sexual dysfunction from low estrogen. Rare but serious: hepatotoxicity (liver function monitoring recommended), skin reactions. The arthralgias are sometimes managed with NSAIDs, low-impact exercise, vitamin D/calcium, and in severe cases switching to a different AI or back to tamoxifen.

Can anastrozole be used in premenopausal women?

No — anastrozole is not effective as monotherapy in premenopausal women because the ovaries continue to produce large amounts of estrogen, overwhelming the peripheral aromatase inhibition. In premenopausal women with HR+ breast cancer, ovarian function suppression (OFS) — using LHRH agonists (goserelin, leuprolide) or bilateral oophorectomy — is required first to eliminate ovarian estrogen production. Only after OFS can an AI be used effectively. Current guidelines from ASCO and ESMO support OFS + anastrozole (or exemestane) for premenopausal women with early HR+ breast cancer who are at higher risk. Without OFS, anastrozole is ineffective and potentially harmful in premenopausal patients by causing a reflex increase in LH/FSH that can stimulate ovarian estrogen production.

How does anastrozole compare to tamoxifen for adjuvant breast cancer treatment?

The landmark ATAC trial (Anastrozole, Tamoxifen, Alone or in Combination) comparing anastrozole vs. tamoxifen for 5 years adjuvant therapy in postmenopausal HR+ early breast cancer demonstrated anastrozole's superiority in disease-free survival and time to recurrence, with lower rates of endometrial cancer, thromboembolic events, and vaginal discharge. However, tamoxifen was associated with lower fracture rates and a trend toward better lipid profiles. Current ASCO guidelines recommend AIs (anastrozole, letrozole, or exemestane) as preferred adjuvant endocrine therapy for postmenopausal HR+ early breast cancer over tamoxifen monotherapy. Tamoxifen remains the standard for premenopausal women not requiring escalated therapy, and for 5-10 years in select patients.

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