A tier · strong
The ATAC trial is one of the largest adjuvant endocrine trials ever run, with 10-year efficacy and safety data, though the entire dataset is confined to postmenopausal breast cancer populations.
anastrozole
Anastrozole is a third-generation non-steroidal aromatase inhibitor approved for hormone receptor-positive breast cancer in postmenopausal women, supported by the 10-year ATAC randomised dataset. Its bone, joint and lipid effects are well documented within that population and are the direct consequence of profound estrogen suppression.
the explanation
Anastrozole blocks the enzyme that converts androgens into estrogen, so estrogen levels fall sharply. It is an approved breast cancer drug, and because estrogen protects bone, lowering it causes more fractures, osteoporosis and joint pain.
regulatory status
Approved prescription medicine
FDA-approved in 1995 for adjuvant treatment of postmenopausal women with hormone receptor-positive early breast cancer, first-line treatment of advanced disease, and second-line treatment after tamoxifen progression. Not approved for use in men, for estrogen control alongside androgen use, or for any fertility indication.
how it works · proposed mechanism
Anastrozole competitively inhibits the aromatase enzyme that converts androgens into estrogens.
Reversible CYP19A1 inhibition
The triazole nitrogen coordinates to the haem iron of aromatase, competing with the natural androgen substrate. The binding is reversible, so enzyme activity returns after the drug is cleared.
Peripheral estrogen suppression
In postmenopausal women most estrogen derives from peripheral aromatisation rather than ovarian output. Blocking that step lowers circulating estradiol by at least 85% at standard dosing.
Downstream skeletal consequence
Estrogen restrains osteoclast activity and maintains bone remodelling balance. Removing it accelerates bone turnover, which is the mechanistic basis for the observed fracture and osteoporosis signal.
what’s reported
evidence shape
28 sourcesThe composition of the cited literature by study type. This describes the shape of the evidence, not its quality — and it does not by itself set the grade.
⚠ the catch
ATAC and its companion trials establish anastrozole's value in postmenopausal women with hormone receptor-positive breast cancer, a population where a recurrence benefit justifies accepting measurable bone loss and joint toxicity under active monitoring. That evidence does not transfer to off-label use for estrogen control alongside anabolic steroids, where there are no randomised trials, no bone density surveillance, and where suppressing estradiol in a young male, who depends on aromatised estrogen for bone mineralisation, lipid handling and libido, is a distinctly different physiological proposition from suppressing it in a postmenopausal woman with cancer.
key published findings
- Human trial (ATAC 10-year analysis, Lancet Oncology 2010, anastrozole n=3,125 vs tamoxifen n=3,116): in the hormone receptor-positive subgroup, disease-free survival favoured anastrozole (HR 0.86, 95% CI 0.78-0.95, P=0.003) and time to recurrence gave HR 0.79 (95% CI 0.70-0.89, P=0.0002), with the absolute difference widening from 2.7% at 5 years to 4.3% at 10 years.
- Human trial (ATAC safety): fractures during active treatment occurred in 451 anastrozole versus 351 tamoxifen patients (OR 1.33, 95% CI 1.15-1.55, P<0.0001), with post-treatment fracture rates comparable between groups.
- Human trial (ATAC safety): endometrial cancer occurred in 6 anastrozole versus 24 tamoxifen patients, showing the opposite hazard trade-off between drug classes.
- Regulatory data (US prescribing information, ATAC adverse reactions): arthralgia 15% vs 11% for tamoxifen, bone fracture 10% vs 7%, osteoporosis 11% vs 7%, and hypercholesterolaemia 9% vs 3.5%.
- Regulatory data (US prescribing information): ischaemic cardiovascular events occurred in 4% of anastrozole versus 3% of tamoxifen patients overall, rising to 17% versus 10% in patients with pre-existing ischaemic heart disease.
limitations of the evidence
- All pivotal randomised data are in postmenopausal women with breast cancer; there are no comparable outcome trials in men or in younger populations.
- Bone mineral density and fracture outcomes were assessed over a defined trial period with monitoring in place, which is not representative of unmonitored off-label use.
- The cardiovascular signal in patients with pre-existing ischaemic heart disease is based on subgroup data and remains difficult to interpret in isolation.
documented safety signals
- Bone mineral density loss and osteoporosis: reported in 11% of anastrozole patients versus 7% on tamoxifen in ATAC labelling.
- Increased fracture risk during active treatment: OR 1.33 (95% CI 1.15-1.55) versus tamoxifen in ATAC.
- Arthralgia and musculoskeletal pain in approximately 15% of patients, a leading cause of treatment discontinuation in practice; associated arthritis, joint stiffness and carpal tunnel syndrome are reported.
- Adverse lipid effects, with hypercholesterolaemia reported in 9% versus 3.5% for tamoxifen.
- Ischaemic cardiovascular events elevated overall (4% vs 3%) and notably in patients with pre-existing ischaemic heart disease (17% vs 10%).
- Hot flushes, fatigue, mood disturbance, depression and insomnia are frequently reported.
- Vaginal dryness, dyspareunia and reduced libido attributable to profound estrogen deprivation.
- Hepatic enzyme elevations and rare hepatitis; rare cutaneous reactions including erythema multiforme and Stevens-Johnson syndrome have been reported post-marketing.
- Embryo-fetal toxicity; contraindicated in pregnancy.
- In men, off-label estrogen suppression risks the recognised consequences of estradiol deficiency, including impaired bone mineralisation, adverse lipid shifts, reduced libido and possible mood and cognitive effects, none of which have been characterised in randomised trials in this population.
identity
| full name | Anastrozole |
| category | Ancillary & Endocrine |
| modality | small molecule |
| formula | C17H19N5 |
| molar mass | 293.37 g/mol |
| cas | 120511-73-1 |
| half-life | Approximately 40-50 hours |
laboratory handling
| storage | Tablets stored at controlled room temperature, approximately 20-25 degrees C, with excursions permitted to 15-30 degrees C, in the manufacturer's container. |
| solubility | White crystalline powder, moderately soluble in water and freely soluble in methanol, ethanol, acetone, tetrahydrofuran and acetonitrile. |
| co-studied with | Discussed in non-clinical settings as an estrogen-control adjunct to androgen use, but no randomised trial has evaluated aromatase inhibition in that context; over-suppression of estradiol in men is itself a documented cause of bone loss, adverse lipids and sexual dysfunction. |
Handling information describes laboratory practice for a research reagent. It is not a preparation guide for use in humans.
the receipts
5 citedUS prescribing information · 2024 · official
ChemicalBook · 2024 · official
Wikipedia drug monograph (referenced compilation) · 2026 · review
The Lancet Oncology · 2010 · randomized
The Lancet · 2005 · randomized
others in Ancillary & Endocrine