COMPOUND

Compounds / Ancillaries

Anastrozole

Anastrozole (Arimidex)

Ancillaries Oral FDA approved, off-label

Non-steroidal aromatase inhibitor. FDA-approved for breast cancer; used off-label on TRT to manage estradiol elevation. Only ~2.6% of men on TRT actually need it.

At a glance

Half-life
~50 h ~50h (FDA PI). The long half-life is why dosing intervals for this compound are measured in days rather than hours.
Routes
Oral
Evidence base
3 studies 2 human · 1 review

Mechanism

Reversible competitive inhibition of aromatase (CYP19A1), the enzyme that converts testosterone and androstenedione to estradiol in peripheral tissue. Dose-dependent reduction of serum E2: trial dosing in TRT patients produced a median E2 drop from 65 to 22 pg/mL. Accelerated BMD loss with long-term use (5-year ATAC data). AI-associated arthralgia driven by estradiol deprivation rather than direct drug toxicity. Does not affect testosterone, cortisol, or other steroidogenic pathways directly.

Dosing

Any amounts shown here are reported from published studies only. 2 of 3 catalogued studies involved human subjects. COMPOUND does not publish its own dosing guidance.

Where a study reports an amount, it appears in that study's entry below, attributed to the source.

Published evidence

  1. Human RCT 2008

    Effect of anastrozole on bone mineral density: 5-year results from the Anastrozole, Tamoxifen, Alone or in Combination (ATAC) trial

    Eastell R, Adams JE, Coleman RE, et al. J Bone Miner Res

    Accelerated BMD loss on anastrozole vs tamoxifen over 5 years: foundational evidence that AIs impact bone long-term.

    PMID 18309940

  2. Human observational 2021

    The utilization and impact of aromatase inhibitor therapy in men with elevated estradiol levels on testosterone therapy

    Punjani N, Bernie H, Salter C, et al. Sex Med

    In a single high-volume center over 14 years, only 2.6% of men on TRT met criteria for AI; those treated with 0.5mg 3x/week saw median E2 drop from 65 → 22 pg/mL.

    PMID 34090245

  3. Review 2022

    Aromatase inhibitor-associated musculoskeletal symptoms: clinical implications and potential mechanisms

    Moseley KF, Naidoo J. Clin Breast Cancer

    Review of AI arthralgia mechanisms: estradiol deprivation is the primary driver. Relevant to both oncology and off-label male TRT contexts.

Reported side effects

Arthralgia / joint pain
Very common Moderate · ~36% in oncology trials; correlates with E2 suppression (<15-20 pg/mL) rather than anastrozole dose directly.
Bone density loss (long-term)
Common Serious · ATAC trial: accelerated BMD loss over 5 years vs tamoxifen. In men, E2 <15 pg/mL impairs bone remodeling. DEXA scan with long-term AI use.
Crashed-E2 symptoms
Common Moderate · Low libido, erectile dysfunction, fatigue, depression, lipid impairment when E2 drops below ~20 pg/mL. The single most common error on TRT AI management.
Hot flashes
Occasional Mild · More common in women on higher oncology doses. Less common in men on TRT micro-dosing.
Lipid panel changes
Occasional Mild · Estradiol is cardioprotective in men; excessive suppression can worsen HDL / triglyceride ratio. Monitor lipid panel with long-term use.

Storage

Oral tablets: room temperature, sealed container, away from moisture.