Drug Monograph
Anastrozole
Arimidex
Pharmacokinetic Profile
Half-Life
~50 hours
Metabolism
Hepatic (~85%); N-dealkylation, hydroxylation, glucuronidation
Protein Binding
40%
Bioavailability
Well absorbed; food does not affect extent
Volume of Distribution
Not fully characterized; widely distributed
Clinical Information
Drug Class
Third-generation non-steroidal aromatase inhibitor
Available Doses
1 mg tablet
Route
Oral
Renal Adjustment
Not required
Hepatic Adjustment
None for mild-moderate; severe not studied
Pregnancy
Contraindicated — may cause fetal harm
Lactation
Do not breastfeed
Schedule / Legal Status
Prescription only (not scheduled)
Generic Available
Yes (since 2010)
Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Hormone receptor-positive early breast cancer — adjuvant therapy | Postmenopausal women | Monotherapy (after surgery ± chemo/radiation) | FDA Approved |
| Locally advanced or metastatic breast cancer — first-line | Postmenopausal women (HR+ or HR-unknown) | Monotherapy | FDA Approved |
| Advanced breast cancer — second-line (post-tamoxifen progression) | Postmenopausal women | Monotherapy | FDA Approved |
Anastrozole is a cornerstone of endocrine therapy for postmenopausal breast cancer. In the adjuvant setting, the landmark ATAC trial demonstrated superiority over tamoxifen in disease-free survival for hormone receptor-positive early disease, with benefits persisting beyond the 5-year treatment period. For advanced disease, anastrozole provides a well-tolerated first-line option with efficacy comparable to tamoxifen and a more favorable side-effect profile regarding thromboembolic and uterine events.
Off-Label Uses
Breast cancer risk reduction in high-risk postmenopausal women (Evidence: High). The IBIS-II trial demonstrated a 49% reduction in breast cancer incidence at 131 months median follow-up with 5 years of anastrozole 1 mg daily versus placebo (HR 0.51, 95% CI 0.39–0.66). Recommended by NICE (UK) and supported by the US Preventive Services Task Force for postmenopausal women at increased risk.
Dosing
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| HR+ early breast cancer — adjuvant after surgery | 1 mg once daily | 1 mg once daily | 1 mg/day | Standard duration is 5 years (ATAC); may be given sequentially after 2–3 years of tamoxifen Can be taken with or without food |
| Locally advanced or metastatic breast cancer — first-line | 1 mg once daily | 1 mg once daily | 1 mg/day | Continue until disease progression Postmenopausal status must be confirmed |
| Advanced breast cancer — second-line after tamoxifen | 1 mg once daily | 1 mg once daily | 1 mg/day | Continue until progression; ER-negative patients rarely respond No washout period required after tamoxifen |
| Breast cancer risk reduction in high-risk postmenopausal women (off-label) | 1 mg once daily | 1 mg once daily | 1 mg/day | 5-year treatment course per IBIS-II protocol Benefits persist beyond treatment period |
Special Populations
| Population | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Renal impairment (any severity) | 1 mg once daily | 1 mg once daily | 1 mg/day | No adjustment needed; only ~10% renally excreted (FDA PI) |
| Hepatic impairment (mild-moderate / stable cirrhosis) | 1 mg once daily | 1 mg once daily | 1 mg/day | No adjustment; clearance ~30% lower but levels remain in normal range (FDA PI) Severe hepatic impairment not studied |
| Elderly (≥65 years) | 1 mg once daily | 1 mg once daily | 1 mg/day | No adjustment; PK unaffected by age (FDA PI) |
Clinical Pearl: Flat Dosing
Anastrozole uses a single fixed dose of 1 mg daily across all indications and populations. Clinical trials tested doses up to 10 mg daily and single doses up to 60 mg without dose-limiting toxicity, but the 1 mg dose achieves maximal estradiol suppression (>80% reduction). There is no titration phase, which simplifies prescribing and adherence counselling.
Pharmacology
Mechanism of Action
Anastrozole is a potent and selective non-steroidal aromatase inhibitor that works by reversibly binding to the aromatase enzyme (cytochrome P450 19A1). This competitive inhibition blocks the conversion of androgens (androstenedione and testosterone) to estrogens (estrone and estradiol) in peripheral tissues. In postmenopausal women, aromatization of adrenal androgens in fat, muscle, liver, and breast tissue is the primary source of circulating estrogen, making peripheral aromatase a high-value therapeutic target. Anastrozole achieves greater than 80% suppression of plasma estradiol levels and has no clinically relevant effect on adrenal corticosteroid synthesis (aldosterone or cortisol), distinguishing it from older, non-selective agents. The parent drug is the primary source of aromatase inhibition; its major circulating metabolite, triazole, lacks pharmacologic activity.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Rapid; Tmax ~2 h (fasted), ~5 h (with food); food does not reduce total absorption | Can be taken with or without food; rapid onset of estradiol suppression |
| Distribution | 40% plasma protein bound; widely distributed | Low protein binding means minimal displacement-type drug interactions |
| Metabolism | Hepatic (~85%); N-dealkylation, hydroxylation, glucuronidation; major metabolite triazole is inactive | Weak CYP inhibitor at clinical doses (Ki ~30× steady-state Cmax); low interaction potential |
| Elimination | t½ ~50 h; ~85% in feces and urine; renal excretion ~10% unchanged | Long half-life supports once-daily dosing; steady state reached at ~7 days |
Side Effects
≥10%
Very Common
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Hot flashes (vasodilation) | 36% | Most common effect; reflects estrogen deprivation; typically persist throughout treatment |
| Asthenia / fatigue | 19% | May be multifactorial in oncology patients; does not usually require dose change |
| Arthritis | 17% | Part of a broader musculoskeletal syndrome; higher than tamoxifen (14%) |
| Pain (general) | 17% | Includes non-specific body pain; often overlaps with underlying disease |
| Arthralgia | 15% | Characteristic AI class effect; may improve over 6 months of continued therapy |
| Pharyngitis | 14% | Similar to tamoxifen (14%); not clearly drug-related |
| Hypertension | 13% | Slightly higher than tamoxifen (11%); monitor BP regularly |
| Depression | 13% | Similar to tamoxifen (12%); screen at regular intervals |
| Nausea | 11% | Usually mild; does not typically require antiemetic therapy |
| Rash | 11% | Maculopapular, usually self-limiting; watch for severe cutaneous reactions |
| Osteoporosis | 11% | Estrogen deprivation accelerates bone loss; co-prescribe calcium/vitamin D |
| Fractures (all types) | 10% | Higher than tamoxifen (7%); increased rate resolves after treatment cessation |
| Back pain | 10% | Part of musculoskeletal symptom complex |
| Insomnia | 10% | Often related to hot flashes and mood disturbance |
| Headache | 10% | Slightly higher than tamoxifen (8%) |
| Peripheral edema | 10% | Mild; similar to tamoxifen (11%) |
| Lymphedema | 10% | Often post-surgical; similar to tamoxifen (11%) |
1–10%
Common
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Hypercholesterolemia | 9% | Notably higher than tamoxifen (3.5%); monitor lipid panel |
| Weight gain | 9% | Similar to tamoxifen; lifestyle counselling recommended |
| Diarrhea | 9% | Usually self-limiting |
| Constipation | 8% | Similar to tamoxifen (8%); manage with fibre and hydration |
| Dizziness | 8% | Advise caution with driving if persistent |
| Breast pain | 8% | Higher than tamoxifen (6%) |
| Dyspnea | 8% | Rule out thromboembolic event if acute onset |
| Bone pain | 7% | Estrogen deprivation effect; evaluate for bone metastases if new or worsening |
| Paresthesia | 7% | Higher than tamoxifen (5%) |
| Anxiety | 6% | Screen alongside depression |
| Myalgia | 6% | Part of the AI-related musculoskeletal syndrome |
| Cataract | 6% | Similar to tamoxifen (7%); annual eye exam recommended |
| Ischemic cardiovascular events | 4% | 17% in those with pre-existing ischemic heart disease (vs 10% tamoxifen) |
| Carpal tunnel syndrome | 2.5% | Significantly higher than tamoxifen (0.7%); distinctive to AIs |
Serious
Serious (Regardless of Frequency)
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Ischemic cardiovascular events (MI, angina) | 4% overall; 17% in pre-existing IHD | During treatment | Cardiovascular risk assessment before initiation; ongoing CV monitoring in high-risk patients; consider risk-benefit in pre-existing IHD |
| Fractures (spine, hip, wrist) | 4% (major sites); 10% all fractures | Throughout treatment; rate normalises post-treatment | Baseline and periodic DEXA; calcium/vitamin D supplementation; consider bisphosphonate if T-score ≤ −2.0 |
| Venous thromboembolic events | 3% | Any time during treatment | Lower than tamoxifen (5%); however, maintain vigilance; discontinue if DVT/PE confirmed |
| Anaphylaxis / angioedema | Very rare (<0.01%) | Any time; usually early in treatment | Emergency care; permanent discontinuation; contraindicated upon re-challenge |
| Stevens-Johnson syndrome / erythema multiforme | Very rare (post-marketing) | Weeks to months | Immediate discontinuation; dermatology referral; do not rechallenge |
| Hepatitis / elevated liver enzymes | Very rare (post-marketing) | Variable | Monitor LFTs if symptoms arise; discontinue if hepatitis confirmed |
| Tendon disorders (rupture, tendonitis) | Rare (post-marketing) | Variable | Discontinue if tendon rupture; orthopaedic referral |
Discontinuation
Discontinuation Rates
ATAC Trial — Adverse Event Withdrawal
11% vs 14% tamoxifen
Top reasons: Musculoskeletal symptoms, hot flashes, fatigue
Real-World AI Adherence
~32% within 2 years
Top reasons: Musculoskeletal pain (24.3%), fatigue, mood changes
| Reason for Discontinuation | Incidence | Context |
|---|---|---|
| Hot flashes | Most common single reason (ATAC) | Fewer withdrawals than tamoxifen despite lower overall incidence |
| Musculoskeletal symptoms | Leading cause in real-world studies | Arthralgia, arthritis, bone pain; may improve with AI switch |
| Treatment-related serious AEs | 5% anastrozole vs 9% tamoxifen | ATAC trial data; anastrozole had better overall tolerability |
Managing AI-Related Musculoskeletal Symptoms
Arthralgia and joint stiffness are the most clinically significant tolerability concern with anastrozole and the leading cause of early discontinuation. Approximately half of patients experiencing these symptoms on anastrozole report improvement within 6 months of continued therapy. For those with intolerable symptoms, switching to an alternative AI (exemestane or letrozole) may be effective — roughly one-third of patients who cannot tolerate one AI can tolerate another. Regular weight-bearing exercise, physiotherapy referral, and short-term NSAID use are reasonable supportive strategies.
Drug Interactions
Anastrozole has a favourable drug interaction profile. Although it weakly inhibits CYP1A2, CYP2C8/9, and CYP3A4 in vitro, the Ki values are approximately 30-fold higher than steady-state plasma concentrations at the 1 mg clinical dose, making clinically significant CYP-mediated interactions unlikely. It has no relevant effect on CYP2A6 or CYP2D6. Despite this, several pharmacodynamic interactions are clinically important.
Major
Tamoxifen
MechanismTamoxifen reduces anastrozole AUC by 27%; opposing pharmacodynamic effects on estrogen pathway
EffectNo additional efficacy benefit from combination over tamoxifen alone (ATAC trial); combination arm was discontinued
ManagementDo not co-administer; use sequentially if switching between agents
FDA PI / ATAC Trial
Major
Estrogen-Containing Products (HRT, combined OC)
MechanismExogenous estrogen directly opposes anastrozole’s mechanism of action
EffectMay completely negate the antitumor efficacy of anastrozole
ManagementDiscontinue all systemic estrogen before starting anastrozole; avoid vaginal estrogens unless carefully assessed by oncology
FDA PI
Minor
Warfarin
MechanismNo pharmacokinetic interaction demonstrated in formal study
EffectNo change in warfarin exposure or anticoagulant activity (PT, aPTT, thrombin time)
ManagementNo dose adjustment needed; standard INR monitoring as per warfarin protocol
FDA PI
Minor
CYP Substrates (general)
MechanismWeak in vitro CYP1A2, CYP2C8/9, CYP3A4 inhibition at Ki values ~30× clinical Cmax
EffectClinically significant interactions with CYP substrates considered unlikely at 1 mg dose
ManagementNo routine dose adjustments for concomitant CYP substrates
FDA PIMonitoring
-
Bone Mineral Density (DEXA)
Baseline, then every 1–2 years
Routine Anastrozole accelerates bone loss by suppressing estrogen. Measure lumbar spine and total hip BMD before starting therapy. If T-score is ≤ −2.0 or if significant decline occurs, consider bisphosphonate co-prescription. All patients should receive calcium (1,000–1,200 mg/day) and vitamin D (800–1,000 IU/day) supplementation. -
Lipid Panel
Baseline, then annually
Routine Elevated total cholesterol was reported in 9% of anastrozole patients vs 3.5% on tamoxifen in the ATAC trial. Manage per NCEP/ACC guidelines for cardiovascular risk-based lipid management. -
Cardiovascular Risk
Baseline assessment, then as clinically indicated
Routine Patients with pre-existing ischemic heart disease had 17% ischemic CV events on anastrozole vs 10% on tamoxifen. Assess cardiovascular risk factors before initiation and weigh risk-benefit carefully in patients with established coronary disease. -
Blood Pressure
Each clinic visit
Routine Hypertension occurred in 13% of patients (vs 11% tamoxifen). Monitor at each visit and treat per guidelines. -
Musculoskeletal Symptoms
Each clinic visit
Routine Actively inquire about joint pain, stiffness, and functional impairment at every visit. Early intervention improves adherence. Consider physiotherapy referral or AI switch if symptoms are intolerable. -
Pregnancy Status
Before initiation
Trigger-based Verify postmenopausal status or negative pregnancy test before starting. Anastrozole can cause fetal harm. Advise effective contraception during treatment and for 3 weeks after the last dose. -
Liver Function
If symptoms arise
Trigger-based Post-marketing reports include hepatitis and elevated transaminases. Check LFTs if patient develops malaise, jaundice, or right upper quadrant pain. -
Mood / Depression Screen
Baseline, then every 3–6 months
Routine Depression was reported in 13% of patients. Routine screening with a validated tool (e.g., PHQ-9) supports early identification and referral.
Contraindications & Cautions
Absolute Contraindications
- Known hypersensitivity to anastrozole or any excipient. Reported reactions include anaphylaxis, angioedema, and urticaria (FDA PI).
- Pregnancy. Anastrozole may cause fetal harm based on its mechanism of action and animal reproductive toxicity data. Embryo-fetal toxicity was observed in rats at exposures 9 times the human AUC (FDA PI).
- Premenopausal women (without ovarian suppression). Anastrozole is not effective in the premenopausal setting because it cannot suppress ovarian estrogen production; compensatory gonadotropin increase would overcome aromatase inhibition.
Relative Contraindications (Specialist Input Recommended)
- Pre-existing ischemic heart disease. The ATAC trial showed ischemic cardiovascular events in 17% of patients with pre-existing IHD on anastrozole vs 10% on tamoxifen. A documented risk-benefit discussion and cardiology input are recommended before prescribing.
- Severe osteoporosis (T-score < −2.5 or prior fragility fractures). Anastrozole accelerates bone loss. If an AI is still the preferred agent, co-prescribe bisphosphonate and ensure close DEXA follow-up.
- Severe hepatic impairment. Anastrozole has not been studied in this population. Use with caution and consider alternative agents.
Use with Caution
- Dyslipidemia. Anastrozole was associated with elevated cholesterol in 9% of patients (vs 3.5% tamoxifen). Monitor lipids and manage per cardiovascular risk guidelines.
- Osteopenia (T-score −1.0 to −2.5). Ensure calcium/vitamin D supplementation, weight-bearing exercise, and serial DEXA monitoring.
- Lactation. No data on presence in human milk. Advise against breastfeeding during treatment and for 2 weeks after the last dose (FDA PI).
FDA Safety Advisory
Ischemic Cardiovascular Events in Pre-Existing Heart Disease
In the ATAC trial, women with pre-existing ischemic heart disease experienced a significantly increased incidence of ischemic cardiovascular events on anastrozole (17%) compared with tamoxifen (10%). The FDA prescribing information advises clinicians to consider the risks and benefits of anastrozole therapy in patients with pre-existing ischemic heart disease. This is listed under Warnings and Precautions (Section 5.1).
FDA Warning — Embryo-Fetal Toxicity
Anastrozole May Cause Fetal Harm
Based on animal findings and its mechanism of action, anastrozole can cause fetal harm when administered to a pregnant woman. Verify pregnancy status prior to initiation. Advise females of reproductive potential to use effective contraception during treatment and for at least 3 weeks after the last dose (FDA PI, Section 5.4).
Patient Counselling
Purpose of Therapy
Anastrozole works by blocking the production of estrogen in postmenopausal women, which removes the hormonal stimulus that helps certain breast cancers grow. In the adjuvant setting, taking anastrozole for 5 years after surgery significantly reduces the risk of cancer recurrence, with protective effects continuing even after the treatment course is completed. For advanced disease, anastrozole slows or stops tumour growth for as long as it remains effective.
How to Take
Take one 1 mg tablet by mouth once daily, at around the same time each day. The tablet can be taken with or without food. If a dose is missed and it is within a few hours of the usual time, take it as soon as remembered. If it is close to the next scheduled dose, skip the missed dose and resume the normal schedule. Do not double up. Treatment typically continues for 5 years in the adjuvant setting, or until disease progression for advanced cancer.
Hot Flashes
Tell patient
Hot flashes are the most common side effect, affecting about 1 in 3 patients. They result from estrogen lowering and are expected. Layered clothing, staying in cool environments, and avoiding known triggers (caffeine, alcohol, spicy food) may help. They tend to remain stable throughout treatment but are a sign the medication is working.
Call prescriber
If hot flashes are so severe that they interfere with sleep, daily activities, or quality of life despite non-pharmacologic measures. Prescription options exist but should be discussed with your oncology team.
Joint Pain & Stiffness
Tell patient
Joint aches, stiffness, or arthralgia occur in about 15–17% of patients and are the most common reason people stop treatment early. Symptoms may improve within 6 months of continued use. Staying physically active with regular gentle exercise can significantly reduce joint symptoms. Over-the-counter pain relief (paracetamol or ibuprofen) may help.
Call prescriber
If joint pain is severe enough to limit movement or daily function, or if swelling or warmth develops in any joint. An alternative medication may be considered.
Bone Health
Tell patient
Anastrozole can decrease bone density because it lowers estrogen levels. Take calcium supplements (1,000–1,200 mg daily) and vitamin D (800–1,000 IU daily) as directed. Engage in regular weight-bearing exercise (walking, light resistance training). Bone density scans will be performed periodically to monitor bone health.
Call prescriber
If you experience a fall or sudden bone pain, especially in the hip, spine, or wrist, as fracture risk is increased during treatment.
Mood Changes
Tell patient
Some patients experience low mood, anxiety, or insomnia while on anastrozole. These effects may be related to hormonal changes. Maintaining social connections, physical activity, and good sleep hygiene can help. Do not hesitate to report mood changes, as effective support is available.
Call prescriber
If you experience persistent sadness, loss of interest in activities, thoughts of self-harm, or anxiety that interferes with daily life.
Pregnancy & Contraception
Tell patient
Anastrozole can harm an unborn baby. If you are of childbearing potential, use effective contraception throughout treatment and for at least 3 weeks after the last dose. Inform your doctor immediately if you think you may be pregnant. Do not breastfeed during treatment or for 2 weeks after the last dose.
Call prescriber
Immediately if pregnancy is suspected or confirmed during treatment.
Adherence & Missed Doses
Tell patient
Taking the tablet consistently every day for the full prescribed course is important for maximum benefit. Studies show that the cancer-preventing benefits of anastrozole continue for years after completing treatment, reinforcing the value of completing the full course. Setting a daily alarm or linking the tablet to an existing routine (e.g., morning coffee) may help.
Call prescriber
If you are considering stopping treatment because of side effects. There are strategies and alternatives that can be explored before discontinuing.
Sources
Regulatory (PI / SmPC)
- ARIMIDEX (anastrozole) Tablets. Full Prescribing Information. ANI Pharmaceuticals / AstraZeneca. Revised 12/2024. FDA Label Primary regulatory source for all dosing, pharmacokinetics, adverse reaction incidence rates, contraindications, and drug interaction data.
- Anastrozole Tablets. DailyMed / National Library of Medicine. DailyMed Entry NLM-hosted prescribing information supporting cross-reference of safety and PK data.
Key Clinical Trials
- Howell A, Cuzick J, Baum M, et al. Results of the ATAC (Arimidex, Tamoxifen, Alone or in Combination) trial after completion of 5 years’ adjuvant treatment for breast cancer. Lancet. 2005;365(9453):60–62. DOI Landmark ATAC 5-year completion results confirming anastrozole superiority over tamoxifen for disease-free survival in HR+ early breast cancer.
- Cuzick J, Sestak I, Baum M, et al. Effect of anastrozole and tamoxifen as adjuvant treatment for early-stage breast cancer: 10-year analysis of the ATAC trial. Lancet Oncol. 2010;11(12):1135–1141. DOI 10-year ATAC follow-up establishing long-term efficacy and confirming that fracture rates normalise after treatment cessation.
- Buzdar A, Howell A, Cuzick J, et al. Comprehensive side-effect profile of anastrozole and tamoxifen as adjuvant treatment for early-stage breast cancer: long-term safety analysis of the ATAC trial. Lancet Oncol. 2006;7(8):633–643. DOI Comprehensive ATAC safety analysis providing treatment-related adverse event rates, withdrawal incidence, and risk-benefit indices.
- Cuzick J, Sestak I, Forbes JF, et al. Anastrozole for prevention of breast cancer in high-risk postmenopausal women (IBIS-II): an international, double-blind, randomised placebo-controlled trial. Lancet. 2014;383(9922):1041–1048. DOI IBIS-II initial results demonstrating 53% breast cancer risk reduction with 5 years of anastrozole in high-risk postmenopausal women.
- Cuzick J, Sestak I, Forbes JF, et al. Use of anastrozole for breast cancer prevention (IBIS-II): long-term results of a randomised controlled trial. Lancet. 2020;395(10218):117–122. DOI IBIS-II long-term results (131-month follow-up) confirming 49% sustained breast cancer reduction and no new late adverse effects.
Guidelines
- National Institute for Health and Care Excellence (NICE). Familial breast cancer: classification, care and managing breast cancer and related risks in people with a family history of breast cancer. Clinical guideline [CG164]. Updated 2019. NICE CG164 NICE guideline recommending anastrozole as a chemoprevention option for high-risk postmenopausal women.
- Burstein HJ, Lacchetti C, Anderson H, et al. Adjuvant endocrine therapy for women with hormone receptor-positive breast cancer: ASCO clinical practice guideline focused update. J Clin Oncol. 2019;37(5):423–438. DOI ASCO guideline on adjuvant endocrine therapy positioning aromatase inhibitors as preferred initial or sequential therapy.
Mechanistic / Basic Science
- Geisler J, Haynes B, Anker G, et al. Influence of letrozole and anastrozole on total body aromatization and plasma estrogen levels in postmenopausal breast cancer patients evaluated in a randomized, cross-over study. J Clin Oncol. 2002;20(3):751–757. DOI Key study quantifying aromatase inhibition and estradiol suppression achieved with third-generation AIs.
Pharmacokinetics / Special Populations
- Plourde PV, Dyroff M, Dukes M. Arimidex: a potent and selective fourth-generation aromatase inhibitor. Breast Cancer Res Treat. 1994;30(1):103–111. DOI Early pharmacological characterisation of anastrozole, establishing its selectivity and potency profile.
- Henry NL, Azzouz F, Desta Z, et al. Predictors of aromatase inhibitor discontinuation as a result of treatment-emergent symptoms in early-stage breast cancer. J Clin Oncol. 2012;30(9):936–942. DOI Prospective study documenting real-world AI discontinuation rates and tolerability of switching between AIs.