Arimidex (Anastrozole)
Anastrozole · Adex · AI
Non-steroidal aromatase inhibitor; used to control estradiol on-cycle. Over-suppression risk.
Key takeaways
- Competitive aromatase inhibitor; estradiol starts falling within about 24 hours of a dose.
- Roughly 47-hour half-life and 80% bioavailability allow small, infrequent dosing.
- Oncology label is 1 mg daily; off-label use runs far lower, guided by estradiol bloodwork.
- Main risk is over-suppression: joint pain, libido loss, and lipid deterioration when estradiol crashes.
Overview
What it is
Arimidex (anastrozole) is a non-steroidal aromatase inhibitor approved for estrogen-receptor-positive breast cancer and used off-label to control estradiol during testosterone therapy or anabolic cycles. It competitively inhibits the aromatase enzyme, cutting the conversion of androgens to estrogens.
Pharmacokinetics
The half-life is roughly 47 hours with about 80% oral bioavailability, and estradiol begins falling within about 24 hours of a dose. Small, infrequent doses reach steady-state suppression because the drug is potent relative to the amount of aromatase in non-oncology users.
What the evidence says
The oncology evidence is strong: the ATAC trial established superiority over tamoxifen for adjuvant breast cancer treatment. Off-label use has no trial support and a different main risk, over-suppression. Estradiol driven too low causes joint pain, libido loss, lipid deterioration, and mood changes, which is why off-label dosing is symptom- and bloodwork-driven rather than scheduled.
Reported dosing protocols
Protocols reported in research literature and clinic/community use — informational context, not a dosing recommendation.
| Use case | Dose | Route | Frequency | Duration |
|---|---|---|---|---|
| Breast cancer (label) | 1 mg | Oral | Once daily | 5 years (adjuvant) |
| Estradiol control on-cycle (community) | 0.25–0.5 mg | Oral | 1–2× weekly | On-cycle, as needed |
Pharmacokinetic summary
- Model tier
- Advanced (t½ + Cmax + Tmax + F)
- Half-life
- 46.8 hours
- Cmax
- 3930 ng/mL
- Tmax
- 0.0 days
- Bioavailability
- 80%
- Typical dose
- 0.25–1 mg
Frequently asked questions
Why is off-label anastrozole dosing so much lower than the label?
The 1 mg oncology label aims for maximal aromatase shutdown in cancer patients. In non-oncology use the goal is reducing estradiol into a normal range, not eliminating it, so reported doses are far lower and guided by bloodwork.
What happens if estradiol gets too low?
Joint pain, libido loss, flat mood, dry membranes, and worsening lipids. Estradiol isn't a waste product: bone, brain, and vascular tissue need some. Over-suppression resolves as the drug clears, given the ~47-hour half-life, usually over several days.
Arimidex vs aromasin — what's the difference?
Anastrozole binds aromatase reversibly; exemestane inactivates it permanently, so its suppression outlasts the drug in blood. Anastrozole lets estradiol rebound faster after stopping, which makes overshoots easier to correct.
References
- PubMed anastrozole PK
- ATAC trial 10-year analysis, Lancet Oncol 2010 — Anastrozole superior to tamoxifen for adjuvant treatment of early breast cancer.
- Anastrozole PK/PD study, JCEM 2009 — Pharmacokinetics and estradiol suppression dynamics behind the plotter model.
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