The Peptide Desk The Peptide Desk

This information is for research and educational purposes only. No medical recommendations. Products are not approved for human use. For health questions, consult a doctor.

Lexicon
PCT & hormone control

Arimidex (Anastrozole)

The standard aromatase inhibitor - keeps estradiol in check on aromatizing cycles.

1 min read 10 sources

To the product To the sources
White tablet bottle with a black TPD label and a purple stripe for anastrozole on a white podium with a glowing rim, next to a large blue 3D molecule model in a bright lab
Approved · in the USA since 1995

Summary

At a glance

Sources checked on 26 Oct 2026

Estradiol in the blood
-85 %

postmenopausal women, six weeks daily, aromatase 97 percent inhibited2

Half-life
50 hours

steady level after about 7 days of daily dosing7

Testosterone in older men
+67 %

after 12 weeks of daily dosing, calculated from the means3

Spine bone density
-1.7 %

older men after one year, slightly up with placebo5

Breast cancer in IBIS-II
-53 %

prevention in women at increased risk, hazard ratio 0.47 versus placebo6

  • Anastrozole is a non-steroidal, reversible aromatase inhibitor. It blocks the enzyme that converts testosterone into estradiol.9
  • In postmenopausal women it inhibited aromatase in the whole body by 97 percent and lowered estradiol by about 85 percent. Letrozole was somewhat stronger in the same study.2
  • In older men with low testosterone, testosterone rose by about two thirds in 12 weeks and estradiol fell by about a third. Over one year spine bone density decreased slightly.35
  • In the large ATAC trial versus tamoxifen, breast cancer recurred less often over 100 months. Fractures, joint complaints and raised cholesterol were more common.17
  • Anastrozole has been approved in the USA since 1995. Everything here is research, not a recommendation.8
Profile

Updated June 2026

How long does it stay in the body?

Share left after one dose, calculated with a half-life of ~2 days

Half-life ~2 days
After 4 days: 25 %
Day 2 · 50 %Day 4 · 25 %Day 6 · 12.5 %Day 10 · 3.1 %

Simplified model without an absorption phase, half-life from the TPD lexicon.

Class
Aromatase inhibitor

non-steroidal, reversible

Use
Oral

1-3x per week

Half-life
about 48 hours
Status
Clinically approved

breast cancer

Sizes
0.5 to 1 mg

2 sizes in the shop

Contents 12 sections
  1. At a glance
  2. What is Anastrozole?
  3. How it works
  4. Use and dosing
  5. What the research shows
  6. Research status
  7. Side effects
  8. Storage
  9. Related compounds
  10. FAQ
  11. Glossary
  12. Sources

What is Anastrozole?

Anastrozole (brand name Arimidex) is a non-steroidal, reversible aromatase inhibitor. It blocks the aromatase enzyme that converts testosterone to estradiol. In anabolic research contexts it is used to control estrogen side effects from aromatizing compounds (e.g. testosterone).

This information is for research and educational purposes only. No medical advice.

Comparison

Estrogen blockers compared

Aromatase inhibitors and estrogen receptor modulators from the TPD lexicon, as described there. Tap a card to open its entry.

Aromatase inhibitor, non-steroidal, reversible Arimidex (Anastrozole) CYP19A1
Aromatase inhibitor, non-steroidal, reversible Letrozole (Femara) CYP19A1 Aromatase inhibitor, steroidal, irreversible Aromasin (Exemestane) CYP19A1 SERM (estrogen receptor modulator) Tamoxifen (Nolvadex) ER SERM (estrogen receptor modulator) Clomid (Clomiphene) ER

Simplified: aromatase inhibitors lower the production of estradiol, SERMs block the receptor and leave estradiol in the blood. Letrozole lowered estrogens somewhat more than anastrozole head to head. 29

How it works

Anastrozole binds reversibly to aromatase and thereby lowers blood estradiol by up to 80%. Less estradiol means less water retention and a lower gynecomastia risk. Reversible means: after stopping, aromatase activity normalizes again.

Where it acts

Where anastrozole acted in studies

Tap a point or a card. Colour = aromatase, grey = observed in studies.

  1. Hormones in the blood

    Whole-body aromatase 97 percent inhibited, estradiol about 85 percent lower2

    CYP19A1
  2. Testes

    Testosterone about two thirds higher in older men3

    Observed in trials
  3. Pituitary

    Less estradiol feedback, LH rose3

    Observed in trials
  4. Bone

    Spine bone density fell in older men, more fractures than with tamoxifen5

    Observed in trials
  5. Joints

    Joint pain more common than with tamoxifen (15 versus 11 percent)7

    Observed in trials
  6. Liver

    About 85 percent of breakdown runs through the liver7

    Observed in trials

Targets in the ChEMBL database9

Compound CHEMBL1399 (Anastrozol): one recorded mechanism, Inhibitor, direct binding.

Aromatase
Gene
CYP19A1
UniProt
P11511
ChEMBL
CHEMBL1978
Action
Inhibitor, direct binding
Transparent glass enzyme with a deep pocket holding a blue 3D molecule, further molecules to the left and right on a light blue background
Anastrozole occupies aromatase. Symbolic image.

Use and dosing

Dosing

Dosing, titration and syringe

Loading the dosing information …

  • Getting started and stepping up, drawn as a staircase
  • Units on the U-100 syringe for every step
  • How long one vial lasts
White tablet bottle with a black TPD label and a purple stripe for anastrozole, next to a few small white tablets and a transparent tablet splitter
Small tablet, according to the entry often split. Symbolic image.

What the research shows

The large ATAC study (Lancet Oncology, 2008) compared anastrozole with tamoxifen in postmenopausal early breast cancer over 100 months and established the long-term efficacy and tolerability of anastrozole. That is the clinical basis from which the estrogen-lowering potency is well documented.

Study results

What the trials measured

Interactive: switch tabs, tap points, show or hide arms. Arms are named relative to each other, no dosing advice.

Hormones in older men with low testosterone3

Change from baseline, in percent (calculated from the means)

Total testosterone3
Twice weekly +31.0%
Daily +66.8%
Bioavailable testosterone3
Twice weekly +54.8%
Daily +109.1%
LH3
Twice weekly +75.6%
Daily +54.9%
Estradiol3
Twice weekly -37.0%
Daily -34.6%

37 men aged 62 to 74. The abstract gives means before and after, for example 343 to 572 ng/dl total testosterone with daily dosing. All changes were significant versus placebo, the abstract does not give placebo values. Estradiol stayed within the normal range.

Spine bone density in older men5

Change after one year, in percent (calculated from the means)

Lumbar spine5
Placebo +0.8%
Daily -1.7%

69 men aged 60 or older. Testosterone rose by about 64 percent after three months and estradiol fell by about 20 percent. Bone density still fell: aromatase inhibition did not improve these men's bones.

Estrogens in the blood, anastrozole versus letrozole2

Mean reduction in percent, six weeks each in crossover

Estradiol2
Anastrozole 84.9%
Letrozole 87.8%
Estrone2
Anastrozole 81.0%
Letrozole 84.3%
Estrone sulfate2
Anastrozole 93.5%
Letrozole 98.0%

12 postmenopausal women with breast cancer. Whole-body aromatase was 97.3 percent inhibited with anastrozole and more than 99.1 percent with letrozole.

Recurrence in hormone-sensitive early breast cancer1

Share with recurrence, in percent

After 5 years1
Tamoxifen 12.5%
Anastrozole 9.7%
After 9 years1
Tamoxifen 21.8%
Anastrozole 17.0%

Postmenopausal women, median follow-up 100 months. The gap kept widening after treatment ended. There was no difference in overall survival.

Breast cancer in women at increased risk6

Share with breast cancer after a median of 5 years, in percent (calculated)

Breast cancer6
Placebo 4.4%
Anastrozole 2.1%

Calculated from 85 of 1,944 women on placebo and 40 of 1,920 on anastrozole. Hazard ratio 0.47. Deaths evenly distributed (17 versus 18).

White, grape-like glass model of glandular tissue in a Petri dish on a microscope stage
The clinical basis comes from breast cancer research. Symbolic image.

Research status

Where the research stands

Anastrozole has been approved since 1995. On ClinicalTrials.gov 493 trials list anastrozole among their interventions, often in cancer treatment. Shown are registered trials related to this entry. ATAC ran under an ISRCTN number. As of 26 Oct 2026. 10

  1. Preclinical Development as a non-steroidal aromatase inhibitor
  2. Phase 1 no registered trial
  3. Phase 2 1 trial, 1 completed
  4. Phase 3 2 trials, 2 completed
  5. Approval in the USA since 1995
3,864 participants in phase 3 Approved: in the USA since 27 December 1995 (NDA 020541) for breast cancer in postmenopausal women, ChEMBL lists anastrozole with phase 4 as the highest phase.8
Show all 3 phase 2 and 3 trials on a timelineHide timeline 3 completed 0 running 1 with results
Phase 2
Phase 3

Bar: start to primary endpoint (planned for running trials). Green = completed, blue = progress until 26 Oct 2026.

Side effects

  • Too-low estradiol: joint pain, libido loss, fatigue, poor sleep - the most common problem with overdosing.
  • Lipids: very low estradiol worsens the cholesterol profile.
  • Bone density: relevant with long-term use.

Tolerability

Side effects in ATAC

Out of 100 participants per group, how many had the event over about 5 years of treatment.

41 of 100 Tamoxifen
36 of 100 Anastrozole
Event TamoxifenAnastrozole
Hot flushes7 41%36%
Arthritis7 14%17%
Joint pain7 11%15%
Osteoporosis7 7%11%
Fracture7 7%10%
Raised cholesterol7 4%9%

Values from the US label (ATAC, about 3,090 women per group). The comparison is tamoxifen, not placebo. Fractures were only more common during treatment, not afterwards.71

Storage

Store dry and protected from light. Out of reach of children.

Three white bottles on a bright shelf next to a folded light blue towel
Store dry, protected from light and out of reach of children. Symbolic image.

Related

Related compounds

Same targets or a different path: all in the TPD lexicon.

Arimidex (Anastrozole)
Letrozole (Femara) Aromatase inhibitor, somewhat stronger in comparison Aromasin (Exemestane) Steroidal, irreversible aromatase inhibitor Tamoxifen (Nolvadex) SERM, comparator in ATAC Clomid (Clomiphene) SERM, blocks the receptor instead of production Enclomiphene (Androxal) SERM, trans-isomer of clomiphene Testosterone Enanthate Testosterone, converted to estradiol via aromatase

FAQ

Common questions about Arimidex (Anastrozole)

Short answers, each backed by the sources below.

What is anastrozole approved for?

In the USA since December 1995 for hormone-sensitive breast cancer in postmenopausal women, after surgery and in advanced disease. It is not approved for men.87

How much does anastrozole lower estradiol?

According to the label by about 70 percent after 24 hours and about 80 percent after 14 days of daily dosing. In a comparative study estradiol fell by 84.9 percent and whole-body aromatase was 97.3 percent inhibited. Letrozole reached 87.8 percent.72

What do studies in men show?

In 37 older men with low testosterone, total testosterone rose by about two thirds (daily) or a third (twice weekly) in 12 weeks. Estradiol fell by about a third in both groups and stayed within the normal range. Lipids, inflammatory markers and insulin action did not change during this time.34

What happens to bone?

In older men spine bone density fell by about 1.7 percent over one year, while it rose slightly with placebo. In ATAC women on anastrozole broke bones more often than on tamoxifen during treatment (2.93 versus 1.90 percent per year), but not after it ended.51

What did the ATAC trial show?

More than 6,000 postmenopausal women with early breast cancer received anastrozole or tamoxifen. After 100 months the cancer recurred less often with anastrozole, after 9 years in 17.0 versus 21.8 percent. There was no difference in overall survival.1

How long does anastrozole act?

The mean half-life is 50 hours and a steady level is reached after about 7 days of daily dosing. The lowering of estradiol lasted up to 6 days after stopping. Food delays absorption but does not change the amount.7

Glossary

Terms explained briefly

Dotted words in the text open these explanations directly.

Aromatase
An enzyme (CYP19A1) that converts testosterone into estradiol and androstenedione into estrone.
Bioavailable testosterone
The part of testosterone that is not tightly bound to SHBG and can act in tissue.
Bone density
Mineral content of bone, a measure of its strength.
Estradiol
The main estrogen. Men need it too, for example for bone, joints and libido.
Half-life
The time until the amount in the blood has fallen by half.
Hazard ratio
The ratio of risk between two groups over time. 0.47 means about 53 percent fewer events.
LH
Luteinising hormone from the pituitary. It signals the testes to produce testosterone.
Reversible
The inhibition wears off once the drug is broken down. The enzyme is not permanently destroyed.
SERM
Selective estrogen receptor modulator. Blocks the receptor in some tissues instead of lowering the production of estradiol.

Evidence

Sources

Show all 10 sourcesHide sources 6 Studies1 Trial registry3 Database checked 26 Oct 2026

Studies

  1. ATAC: anastrozole versus tamoxifen after 100 months

    ATAC Trialists' Group, Lancet Oncology, 2008

    DOI 10.1016/S1470-2045(07)70385-6 PMID 18083636
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  2. Aromatase inhibition and estrogens: anastrozole versus letrozole

    Geisler et al., Journal of Clinical Oncology, 2002

    DOI 10.1200/JCO.2002.20.3.751 PMID 11821457
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  3. Older men with low testosterone: hormones after 12 weeks

    Leder et al., Journal of Clinical Endocrinology and Metabolism, 2004

    DOI 10.1210/jc.2003-031467 PMID 15001605
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  4. Lipids and inflammatory markers in older men

    Dougherty et al., Clinical Endocrinology, 2005

    DOI 10.1111/j.1365-2265.2005.02205.x PMID 15670201
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  5. Bone density in older men after one year

    Burnett-Bowie et al., Journal of Clinical Endocrinology and Metabolism, 2009

    DOI 10.1210/jc.2009-0739 PMID 19820017
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  6. IBIS-II: prevention of breast cancer at increased risk

    Cuzick et al., The Lancet, 2014

    DOI 10.1016/S0140-6736(13)62292-8 PMID 24333009
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed

Trial registry

  1. Trials with anastrozole on ClinicalTrials.gov

    ClinicalTrials.gov, U.S. National Library of Medicine

    Checked 26 Oct 2026 via ClinicalTrials.gov API v2

Database

  1. US prescribing information for anastrozole (effects, side effects, pharmacokinetics)

    DailyMed, U.S. National Library of Medicine (NDA 020541, Stand 2026)

    Checked 26 Oct 2026 via openFDA Label API, DailyMed
  2. US approval of anastrozole (NDA 020541, 27 Dec 1995)

    Drugs@FDA, U.S. Food and Drug Administration

    Checked 26 Oct 2026 via openFDA-API
  3. Anastrozole in ChEMBL (mechanism and approval)

    ChEMBL, EMBL-EBI (CHEMBL1399)

    Checked 26 Oct 2026 via ChEMBL

In the shop

Arimidex (Anastrozole) in the catalog

Sizes, prices and shipping times are on the product page.

To the product