Anastrozole

What it is

Anastrozole is a synthetic non-steroidal aromatase inhibitor supplied as a 1 mg tablet, originally branded Arimidex. It blocks the aromatase enzyme that, after the menopause, makes most of the body's oestrogen by converting adrenal androgens into oestrone and oestradiol. It is a prescription medicine in the United States for adjuvant and advanced hormone-receptor-positive breast cancer. The US label carries no boxed warning (version dated 2 March 2026).

What the research says

Anastrozole lowers oestrogen to near-zero in postmenopausal women, which starves oestrogen-driven breast cancer. The evidence is unusually complete. In early breast cancer, a patient-level meta-analysis of 31,920 women found ten-year breast cancer death of 12.1% on an aromatase inhibitor against 14.2% on tamoxifen - real, but a 2.1 percentage point difference, not a transformation. The ATAC trial that established anastrozole showed fewer recurrences at ten years yet almost no difference in overall mortality. For prevention in high-risk healthy women, IBIS-II halved breast cancer incidence and the effect lasted after the tablets stopped, with 29 women treated to prevent one cancer - but no reduction in deaths has been shown. The costs are oestrogen-deprivation costs: joint pain, bone loss and fractures, hot flushes, and more ischaemic cardiac events in women who already have heart disease.

Evidence grade: Well established.

How it works

Drug class: Non-steroidal aromatase inhibitor (third generation)

Many breast cancers grow faster when oestrogen is present. After the menopause the ovaries stop making oestrogen, and almost all of it is produced instead by the aromatase enzyme in fat, muscle and the tumour itself, converting adrenal androgens into oestrone and oestradiol. Anastrozole blocks aromatase, so oestrogen levels fall and oestrogen-driven tumours lose their fuel. (Source 1)

What it is used for

• Five years of anastrozole reduced recurrence compared with tamoxifen and, in the pooled patient-level data, lowered ten-year breast cancer mortality from 14.2% to 12.1%. ATAC itself found almost no difference in overall mortality (hazard ratio 0.95, p=0.4). Evidence: established. (Source 2)
• An approved indication in the United States. The group's evidence review here rests on the label rather than on an outcome trial we could reach, so the strength of the advanced-disease evidence is not characterised from primary trials in this batch. Evidence: limited. (Source 3)
• Approved, with the label noting that women with ER-negative disease and those who did not respond to tamoxifen rarely responded to anastrozole. Evidence: limited. (Source 3)
• Not an approved US indication but supported by a large randomised placebo-controlled trial: breast cancer incidence roughly halved over eleven years, number needed to treat 29. No reduction in death has been demonstrated. Evidence: established. (Source 4)
• Explicitly off-label. The available evidence is retrospective and uncontrolled: semen parameters and hormone levels improved from baseline in 105 men, but there was no placebo arm, so natural variation cannot be excluded. Evidence: limited. (Source 5)
• A 12-month single-arm study in 28 girls found no statistically significant effect on vaginal bleeding days or bone-age advance, and the label records the absence of a control group as limiting even the one change seen. Evidence: not-supported. (Source 6)

Interactions

• Tamoxifen (pharmacokinetic study): Taking tamoxifen with anastrozole lowers anastrozole blood levels by about a quarter and added nothing to efficacy in the ATAC trial, so the combination arm was abandoned and the label says the two should not be given together. (Source 7)
• Oestrogen-containing products, including hormone replacement therapy and oestrogen-containing supplements (label): Anastrozole works by removing oestrogen, so taking oestrogen in any form works directly against it. The label states these should not be used together. (Source 7)
• Calcium and vitamin D, bisphosphonates and other bone treatments (label): Not an interaction that reduces anastrozole's effect, but a consequence of it: because oestrogen loss reduces bone mineral density, bone protection becomes relevant. The label advises considering bone density monitoring rather than specifying a supplement. (Source 8)

Stopping it

• There is no withdrawal syndrome described for anastrozole. What the trials show is that the harms are tied to the period of use: in ATAC the fracture excess present during treatment was gone afterwards. (Source 9)
• The benefit, by contrast, carries over: in prevention, the reduction in breast cancer remained significant after the five years of tablets ended, and the IBIS-II authors report no new late side effects. (Source 10)
• Serious adverse events also settled after treatment stopped: in ATAC the advantage of anastrozole over tamoxifen on treatment-related serious adverse events disappeared once treatment ended. (Source 9)

What goes wrong

Fractures were a third more common on anastrozole than tamoxifen while the drug was being taken, and the excess stopped when treatment stopped. (Source 9)

• Randomized trial, High certainty.
• Size: Safety population 6,186 (anastrozole 3,092, tamoxifen 3,094)
• Who: Postmenopausal women with early-stage breast cancer.
• How long: Five years of treatment plus post-treatment follow-up to a median of 120 months.
• Result: Fractures during active treatment 451 on anastrozole vs 351 on tamoxifen (odds ratio 1.33, 95% CI 1.15-1.55; p<0.0001); after treatment 110 vs 112 (odds ratio 0.98, 0.74-1.30; p=0.9). Treatment-related serious adverse events were fewer on anastrozole (223 vs 369; OR 0.57, 0.48-0.69)
• Funding: industry-funded (AstraZeneca-sponsored ATAC trial)

Fractures were more frequent during active treatment in patients receiving anastrozole than those receiving tamoxifen (451 vs 351; OR 1·33, 95% CI 1·15-1·55; p<0·0001), but were similar in the post-treatment follow-up period (110 vs 112; OR 0·98, 95% CI 0·74-1·30; p=0·9).

The same patient-level meta-analysis found about half again as many bone fractures on an aromatase inhibitor as on tamoxifen. (Source 11)

• Meta-analysis, High certainty.
• Size: 31,920 postmenopausal women across the randomised trials.
• Who: Postmenopausal women with oestrogen-receptor-positive early breast cancer.
• How long: Five-year fracture risk.
• Result: Five-year fracture risk 8.2% with an aromatase inhibitor vs 5.5% with tamoxifen (rate ratio 1.42, 95% CI 1.28-1.57), an absolute excess of 2.7 percentage points, i.e. about one extra fracture for every 37 women treated for five years. Endometrial cancer was lower (0.4% vs 1.2%; RR 0.33, 0.21-0.51) and non-breast-cancer mortality similar.
• Funding: independent (Cancer Research UK and Medical Research Council)

There were fewer endometrial cancers with aromatase inhibitors than tamoxifen (10-year incidence 0·4% vs 1·2%; RR 0·33, 0·21-0·51) but more bone fractures (5-year risk 8·2% vs 5·5%; RR 1·42, 1·28-1·57)

In women who already had ischaemic heart disease, ischaemic cardiovascular events were considerably more common on anastrozole than on tamoxifen. (Source 8)

• Official position, Certainty not rated.
• Size: Subgroup of the 6,186-patient ATAC safety population with pre-existing ischaemic heart disease.
• Who: Postmenopausal women with early breast cancer and pre-existing ischaemic heart disease.
• How long: Five years of adjuvant treatment.
• Result: 17% of patients on anastrozole versus 10% on tamoxifen had ischaemic cardiovascular events in this subgroup. In the whole population there was no statistical difference (4% vs 3%); angina 2.3% vs 1.6%, myocardial infarction 1.2% vs 1.1%. Label version dated 2 March 2026.
• Funding: not applicable (regulatory label)

In women with pre-existing ischemic heart disease, an increased incidence of ischemic cardiovascular events was observed with ARIMIDEX in the ATAC trial (17% of patients on ARIMIDEX and 10% of patients on tamoxifen).

Joint pain, arthritis, osteoporosis, fractures and hot flushes all occur in at least one in ten women taking anastrozole. (Source 12)

• Official position, Certainty not rated.
• Size: ATAC early breast cancer study population.
• Who: Postmenopausal women taking anastrozole for early breast cancer.
• How long: Five years.
• Result: Adverse reactions with an incidence of 10% or more in the ATAC study included hot flashes, asthenia, arthritis, pain, arthralgia, pharyngitis, hypertension, depression, nausea and vomiting, rash, osteoporosis, fractures, back pain, insomnia, headache, peripheral edema and lymphedema. Label version dated 2 March 2026.
• Funding: not applicable (regulatory label)

the most common (occurring with an incidence of ≥10%) side effects occurring in women taking ARIMIDEX included: hot flashes, asthenia, arthritis, pain, arthralgia, pharyngitis, hypertension, depression, nausea and vomiting, rash, osteoporosis, fractures, back pain, insomnia, headache, peripheral edema and lymphedema

What the evidence supports

Over ten years, five years of anastrozole beat five years of tamoxifen on disease-free survival and recurrence in postmenopausal early breast cancer. (Source 2)

• Randomized trial, High certainty.
• Size: 6,241 randomised (anastrozole 3,125, tamoxifen 3,116); 5,216 hormone-receptor-positive.
• Who: Postmenopausal women with early-stage breast cancer.
• How long: Median follow-up 120 months.
• Result: Whole population: disease-free survival hazard ratio 0.91 (95% CI 0.83-0.99, p=0.04), time to recurrence 0.84 (0.75-0.93, p=0.001). Hormone-receptor-positive: disease-free survival 0.86 (0.78-0.95, p=0.003), time to recurrence 0.79 (0.70-0.89, p=0.0002); absolute difference in time to recurrence 2.7% at 5 years and 4.3% at 10 years.
• Funding: industry-funded (AstraZeneca-sponsored ATAC trial)

For hormone-receptor-positive patients, the results were also significantly in favour of the anastrozole group for disease-free survival (HR 0·86, 95% CI 0·78-0·95; p=0·003), time to recurrence (0·79, 0·70-0·89; p=0·0002), and time to distant recurrence (0·85, 0·73-0·98; p=0·02).

Pooling individual data on 31,920 women, five years of an aromatase inhibitor lowered ten-year breast cancer death from 14.2% to 12.1% compared with five years of tamoxifen. (Source 13)

• Meta-analysis, High certainty.
• Size: 31,920 postmenopausal women with oestrogen-receptor-positive early breast cancer.
• Who: Postmenopausal women in randomised trials of aromatase inhibitor versus tamoxifen regimens.
• How long: Ten-year outcomes.
• Result: Ten-year breast cancer mortality 12.1% with an aromatase inhibitor vs 14.2% with tamoxifen (rate ratio 0.85, 95% CI 0.75-0.96; 2p=0.009), an absolute difference of 2.1 percentage points. Recurrence rate ratio 0.64 (0.52-0.78) in years 0-1 and 0.80 (0.68-0.93) in years 2-4.
• Funding: independent (Cancer Research UK and Medical Research Council)

10-year breast cancer mortality was lower with aromatase inhibitors than tamoxifen (12·1% vs 14·2%; RR 0·85, 0·75-0·96; 2p=0·009).

In healthy postmenopausal women at high risk, five years of anastrozole roughly halved the number who developed breast cancer. (Source 4)

• Randomized trial, High certainty.
• Size: 3,864 women (1,920 anastrozole, 1,944 placebo)
• Who: Postmenopausal women aged 40-70 at increased risk of breast cancer, 18 countries.
• How long: Median follow-up 5.0 years at first analysis.
• Result: 40 of 1,920 (2%) on anastrozole vs 85 of 1,944 (4%) on placebo developed breast cancer (hazard ratio 0.47, 95% CI 0.32-0.68, p<0.0001). Predicted cumulative incidence at 7 years 2.8% vs 5.6%. Deaths 18 vs 17 (p=0.836)
• Funding: part industry-funded (Cancer Research UK, NHMRC Australia, Sanofi-Aventis and AstraZeneca)

After a median follow-up of 5·0 years (IQR 3·0-7·1), 40 women in the anastrozole group (2%) and 85 in the placebo group (4%) had developed breast cancer (hazard ratio 0·47, 95% CI 0·32-0·68, p<0·0001).

The preventive effect persisted after the five years of tablets ended, with 29 women needing treatment to prevent one breast cancer over about eleven years. (Source 14)

• Randomized trial, High certainty.
• Size: 3,864 women (1,920 anastrozole, 1,944 placebo)
• Who: Postmenopausal women at increased risk of breast cancer.
• How long: Median follow-up 131 months (IQR 105-156)
• Result: 85 vs 165 breast cancers, hazard ratio 0.51 (95% CI 0.39-0.66, p<0.0001); 0.39 (0.27-0.58) in the first five years and still 0.64 (0.45-0.91, p=0.014) afterwards. Ductal carcinoma in situ reduced 59% (0.41, 0.22-0.79). The authors state the number needed to treat to prevent one breast cancer fell to 29.
• Funding: part industry-funded (Cancer Research UK, NHMRC Australia, Breast Cancer Research Foundation, Sanofi Aventis and AstraZeneca)

Limit of this finding: The published abstract prints the confidence interval for ductal carcinoma in situ in women known to be oestrogen receptor-positive as 0.22, 0.78-0.65, which cannot be right: the lower end of the range is bigger than the upper end. That is a mistake in the paper itself, not in the figures quoted here. Read nothing into that particular range - it does not tell you how precise that subgroup result was - and it does not affect the main trial numbers above, which the paper reports consistently.

After a median follow-up of 131 months (IQR 105–156), a 49% reduction in breast cancer was observed for anastrozole (85 vs 165 cases, hazard ratio [HR] 0·51, 95% CI 0·39–0·66, p<0·0001).

What the evidence does not support

Despite fewer recurrences, ATAC found essentially no difference in overall mortality between anastrozole and tamoxifen at ten years. (Source 9)

• Randomized trial, High certainty.
• Size: 6,241 randomised patients.
• Who: Postmenopausal women with early-stage breast cancer.
• How long: Median follow-up 120 months.
• Result: Overall mortality hazard ratio 0.95 (95% CI 0.84-1.06; p=0.4); deaths after recurrence in the hormone-receptor-positive subgroup 0.87 (0.74-1.02; p=0.09)
• Funding: industry-funded (AstraZeneca-sponsored ATAC trial)

but there was little difference in overall mortality (0·95, 95% CI 0·84-1·06; p=0·4)

Prevention with anastrozole has not been shown to reduce deaths, including deaths from breast cancer. (Source 14)

• Randomized trial, Moderate certainty.
• Size: 3,864 women (1,920 anastrozole, 1,944 placebo)
• Who: Postmenopausal women at increased risk of breast cancer.
• How long: Median follow-up 131 months.
• Result: Deaths 69 on anastrozole vs 70 on placebo (hazard ratio 0.96, 95% CI 0.69-1.34, p=0.82); breast cancer deaths two vs three. The authors say further follow-up is needed to assess the effect on breast cancer mortality.
• Funding: part industry-funded (Cancer Research UK, NHMRC Australia, Breast Cancer Research Foundation, Sanofi Aventis and AstraZeneca)

Limit of this finding: The published abstract prints the confidence interval for ductal carcinoma in situ in women known to be oestrogen receptor-positive as 0.22, 0.78-0.65, which cannot be right: the lower end of the range is bigger than the upper end. That is a mistake in the paper itself, not in the figures quoted here. Read nothing into that particular range - it does not tell you how precise that subgroup result was - and it does not affect the main trial numbers above, which the paper reports consistently.

No significant difference in deaths was observed overall (69 vs 70, HR 0·96, 95% CI 0·69–1·34, p=0·82) or for breast cancer (two anastrozole vs three placebo).

In the prevention trial the fracture excess did not persist: over eleven years there was no overall excess of fractures, heart attacks or strokes. (Source 15)

• Randomized trial, Moderate certainty.
• Size: 3,864 women (1,920 anastrozole, 1,944 placebo)
• Who: Postmenopausal women at increased risk of breast cancer, no breast cancer at entry.
• How long: Median follow-up 131 months.
• Result: Fractures 380 vs 373 overall (odds ratio 1.04, 95% CI 0.88-1.22); 198 vs 186 during treatment (1.09, 0.87-1.35) and 182 vs 187 afterwards (0.98, 0.79-1.23). Myocardial infarctions evenly split; transient ischaemic attacks and strokes non-significantly more common on anastrozole (46 vs 36, p=0.24)
• Funding: part industry-funded (Cancer Research UK, NHMRC Australia, Breast Cancer Research Foundation, Sanofi Aventis and AstraZeneca)

In particular, there was no excess of fractures overall (380 vs 373, OR 1·04, 95% CI 0·88–1·22).

Anastrozole failed in girls with McCune-Albright syndrome and precocious puberty, an off-label paediatric use. (Source 6)

• Case series, Very low certainty.
• Size: 28 girls aged 2 to under 10 years.
• Who: Girls with typical or atypical McCune-Albright syndrome, precocious puberty and vaginal bleeding.
• How long: 12 months.
• Result: No statistically significant reduction in frequency of vaginal bleeding days or in the rate of increase of bone age; no clinically significant changes in Tanner staging, ovarian or uterine volume or predicted adult height. Growth rate fell from 7.9 to 6.5 cm/year but there was no control group. Label version dated 2 March 2026.
• Funding: not applicable (regulatory label reporting a single-arm study)

Limit of this finding: The label's own numbers do not add up here: it describes the 28 girls as having typical (27/28) or atypical (1/27) McCune-Albright syndrome, where the second figure should also be out of 28. The quote is reproduced exactly as the label prints it. The study also had no comparison group, and the label itself says that this makes it impossible to put the one change it did find, a small drop in growth rate, down to the drug rather than to the condition.

Compared to pre-treatment data there were no on-treatment statistically significant reductions in the frequency of vaginal bleeding days, or in the rate of increase of bone age

Where the evidence is mixed

The anti-recurrence advantage of aromatase inhibitors applies only while the drugs are actually different; after that the curves run parallel. (Source 11)

• Meta-analysis, High certainty.
• Size: 31,920 postmenopausal women across the randomised trials.
• Who: Postmenopausal women with oestrogen-receptor-positive early breast cancer.
• How long: Ten-year follow-up.
• Result: Recurrence rate ratio 0.70 (95% CI 0.64-0.77) while treatments differed, but 0.93 (0.86-1.01; 2p=0.08) afterwards. All periods combined, breast cancer mortality rate ratio 0.86 (0.80-0.94; 2p=0.0005) and all-cause mortality 0.88 (0.82-0.94; 2p=0.0003)
• Funding: independent (Cancer Research UK and Medical Research Council)

Aggregating all three types of comparison, recurrence RRs favoured aromatase inhibitors during periods when treatments differed (RR 0·70, 0·64-0·77), but not significantly thereafter (RR 0·93, 0·86-1·01; 2p=0·08).

For the off-label use in subfertile men, improvements in sperm measures were reported but without any control group. (Source 5)

• Cohort study, Very low certainty.
• Size: 105 subfertile men (62 with testosterone-to-oestradiol ratio <10, 43 with ratio >10)
• Who: Men with oligoasthenozoospermia, azoospermia or cryptozoospermia attending one andrology clinic in Shanghai.
• How long: Three months of treatment.
• Result: Significant increases from baseline in FSH, LH, total testosterone, testosterone-to-oestradiol ratio, semen volume, sperm concentration, total sperm count, progressive motility and total progressive motility count in both groups, with no significant difference between groups. Retrospective, single-arm, no placebo.
• Funding: not stated.

The majority of subfertile men with and without abnormal T/E2 ratios responded to anastrozole treatment with significantly improved semen parameters and sex hormone levels.

Where the research disagrees

Whether the advantage of an aromatase inhibitor over tamoxifen is durable or only lasts while the drugs differ

• ATAC investigators (Lancet Oncology, 2010), randomised trial, 6,241 patients, median 120 months: "recurrence rates remained significantly lower on anastrozole than tamoxifen after treatment completion (HR 0·81, 95% CI 0·67-0·98; p=0·03), although the carryover benefit was smaller after 8 years" (Source 2)
• Early Breast Cancer Trialists' Collaborative Group (Lancet, 2015), patient-level meta-analysis, 31,920 women: "Aromatase inhibitors reduce recurrence rates by about 30% (proportionately) compared with tamoxifen while treatments differ, but not thereafter." (Source 16)

Whether anastrozole for prevention is worth it given that no mortality benefit has been shown

• IBIS-II investigators (Lancet, 2020), randomised placebo-controlled trial, 3,864 women, median 131 months: "The number needed to treat to prevent one breast cancer has been reduced to 29." (Source 10)
• The same trial's own mortality data, same randomised trial, pre-specified secondary outcome: "No significant difference in deaths was observed overall (69 vs 70, HR 0·96, 95% CI 0·69–1·34, p=0·82) or for breast cancer (two anastrozole vs three placebo)." (Source 14)

How much

• Reference intake: There is no reference intake for a prescription drug. Dosing is set by the prescriber; the US label gives a single dose for every indication: one 1 mg tablet once a day, continued until tumour progression in advanced disease (label version 2 March 2026). (Source 3)
• Upper limit: No higher dose is licensed: the label's dose is the maximum, one 1 mg tablet daily. Doubling or trebling it has no approved basis (label version 2 March 2026). (Source 3)
• Studied: ATAC gave anastrozole 1 mg orally every day for five years, against tamoxifen 20 mg, as adjuvant treatment. (Source 2)
• Studied: IBIS-II gave 1 mg oral anastrozole or matching placebo every day for five years to high-risk postmenopausal women. (Source 4)
• Studied: The off-label male subfertility series used 1 mg anastrozole daily for three months. (Source 5)
• Studied: The McCune-Albright study gave 1 mg daily for 12 months to 28 girls aged 2 to under 10. (Source 6)

A common belief, and what the research shows

The belief: Anastrozole is a far better drug than tamoxifen, so switching means a much better chance of surviving breast cancer.

What the research shows: The difference is real but modest, and it is mostly about recurrence rather than survival. In the 31,920-woman patient-level meta-analysis, "10-year breast cancer mortality was lower with aromatase inhibitors than tamoxifen (12·1% vs 14·2%; RR 0·85, 0·75-0·96; 2p=0·009)." That is about two women in a hundred. In the ATAC trial itself "there was little difference in overall mortality (0·95, 95% CI 0·84-1·06; p=0·4)". And the trade is not free: five-year fracture risk was 8.2% on an aromatase inhibitor against 5.5% on tamoxifen.

Questions and answers

What is it?

Anastrozole is a synthetic non-steroidal aromatase inhibitor, taken as one 1 mg tablet a day. It is a prescription medicine, originally branded Arimidex. It is not a hormone itself; it blocks the enzyme that makes oestrogen. (Source 3)

What does it do in the body?

After the menopause, most oestrogen is made outside the ovaries by the aromatase enzyme converting adrenal androgens into oestrone and oestradiol. Anastrozole blocks that enzyme, so blood oestrogen falls steeply. Because many breast cancers are driven by oestrogen, starving them of it slows or prevents their growth. (Source 1)

Is it good or bad for you?

Both, in measurable amounts. Pooling 31,920 women, five years of an aromatase inhibitor lowered ten-year breast cancer death from 14.2% to 12.1% compared with tamoxifen, while raising five-year fracture risk from 5.5% to 8.2%. In the same analysis endometrial cancer was lower on an aromatase inhibitor. Which way the balance falls depends on a woman's recurrence risk, her bones and her heart. (Source 11)

How do you get more of it?

Anastrozole is available only on prescription and no food or supplement contains it. The dose used in every major trial, for both treatment and prevention, is the same: 1 mg once a day, which is also the label dose. In IBIS-II it was taken for five years. Dose and duration are decided by the prescriber. (Source 10)

If it is harmful, what reduces it?

The way to reverse anastrozole's effects is to stop it, which is a prescriber's decision. The evidence is that the harms are largely confined to the treatment period: in ATAC the fracture excess disappeared after treatment ended (110 vs 112 fractures, odds ratio 0.98), and in IBIS-II there was no excess of fractures over eleven years. Bone loss itself may need separate assessment and treatment. (Source 9)

Why might someone be low in it or missing it?

Anastrozole is only for postmenopausal women with hormone-receptor-positive or receptor-unknown breast cancer, or at high risk of it; it does not apply to premenopausal women, whose ovaries make oestrogen by a different route, and the label notes that women with ER-negative disease rarely respond. Many women also stop it: in ATAC hot flushes were the commonest reason for discontinuation in both arms. (Source 3)

Which whole foods contain it or feed it?

No whole food contains anastrozole, and no food is known to supply it. What matters is the opposite direction: oestrogen-containing products must not be taken with it because they undo its action, and the label warns that tamoxifen lowers anastrozole blood levels by 27% without adding benefit. (Source 7)

What happens if you do not have it?

Anastrozole is a treatment, not a nutrient, so the question is what happens without endocrine treatment. The patient-level meta-analysis estimates that five years of an aromatase inhibitor reduces ten-year breast cancer death by about 15% relative to tamoxifen and by about 40% relative to no endocrine treatment at all. In prevention, 4% of untreated high-risk women developed breast cancer within five years, against 2% on anastrozole. (Source 16)

How can you test for it?

There is no routine blood test for anastrozole itself. What is monitored is its main consequence for the body: because it strips out oestrogen, bone mineral density falls, and the label advises considering bone density monitoring (DXA scanning). Oestradiol assays can confirm oestrogen suppression but the label does not require them. (Source 8)

References

1. DailyMed / ANI Pharmaceuticals. ARIMIDEX- anastrozole tablet (US prescribing information, Mechanism of Action). 2026. Read the source
2. The Lancet Oncology. Effect of anastrozole and tamoxifen as adjuvant treatment for early-stage breast cancer: 10-year analysis of the ATAC trial.. 2010. PMID 21087898, DOI 10.1016/s1470-2045(10)70257-6. Read the source
3. DailyMed / ANI Pharmaceuticals. ARIMIDEX- anastrozole tablet (US prescribing information, Indications and Usage). 2026. Read the source
4. The Lancet. Anastrozole for prevention of breast cancer in high-risk postmenopausal women (IBIS-II): an international, double-blind, randomised placebo-controlled trial.. 2014. PMID 24333009, DOI 10.1016/s0140-6736(13)62292-8. Read the source
5. Translational Andrology and Urology. The efficacy of anastrozole in subfertile men with and without abnormal testosterone to estradiol ratios. 2022. PMID 36217397, DOI 10.21037/tau-22-95. Read the source
6. DailyMed / ANI Pharmaceuticals. ARIMIDEX- anastrozole tablet (US prescribing information, McCune-Albright Syndrome study). 2026. Read the source
7. DailyMed / ANI Pharmaceuticals. ARIMIDEX- anastrozole tablet (US prescribing information, Drug Interactions). 2026. Read the source
8. DailyMed / ANI Pharmaceuticals. ARIMIDEX- anastrozole tablet (US prescribing information, Warnings and Precautions). 2026. Read the source
9. The Lancet Oncology. Effect of anastrozole and tamoxifen as adjuvant treatment for early-stage breast cancer: 10-year analysis of the ATAC trial (fractures and serious adverse events).. 2010. PMID 21087898, DOI 10.1016/s1470-2045(10)70257-6. Read the source
10. The Lancet. Use of anastrozole for breast cancer prevention (IBIS-II): long-term results (research in context). 2020. PMID 31839281, DOI 10.1016/S0140-6736(19)32955-1. Read the source
11. The Lancet. Aromatase inhibitors versus tamoxifen in early breast cancer: patient-level meta-analysis of the randomised trials (aggregate results, fractures and interpretation).. 2015. PMID 26211827, DOI 10.1016/s0140-6736(15)61074-1. Read the source
12. DailyMed / ANI Pharmaceuticals. ARIMIDEX- anastrozole tablet (US prescribing information, Adverse Reactions). 2026. Read the source
13. The Lancet. Aromatase inhibitors versus tamoxifen in early breast cancer: patient-level meta-analysis of the randomised trials.. 2015. PMID 26211827, DOI 10.1016/s0140-6736(15)61074-1. Read the source
14. The Lancet. Use of anastrozole for breast cancer prevention (IBIS-II): long-term results of a randomised controlled trial. 2020. PMID 31839281, DOI 10.1016/S0140-6736(19)32955-1. Read the source
15. The Lancet. Use of anastrozole for breast cancer prevention (IBIS-II): long-term results (adverse events). 2020. PMID 31839281, DOI 10.1016/S0140-6736(19)32955-1. Read the source
16. The Lancet. Aromatase inhibitors versus tamoxifen in early breast cancer: patient-level meta-analysis of the randomised trials (interpretation and methods).. 2015. PMID 26211827, DOI 10.1016/s0140-6736(15)61074-1. Read the source