Anastrozole vs Exemestane: Which Aromatase Inhibitor to Choose?
Estrogen control on cycle is crucial. Anastrozole (Arimidex) suppresses aromatase reversibly; Exemestane (Aromasin) deactivates it permanently. We explain when to use each and how to avoid crashing your estradiol.
Why Estrogen Management Is Non-Negotiable on Cycle
Testosterone aromatises (converts) to estradiol (E2) via the aromatase enzyme, primarily in adipose tissue. On supraphysiological testosterone doses, estradiol rises proportionally. Elevated E2 causes: water retention and bloating, gynecomastia (breast tissue growth), mood instability, loss of libido, and difficulty with vascularity. However, crashing E2 to near-zero (over-suppression with aggressive AI use) is equally problematic: joint pain, erectile dysfunction, severe mood depression, and impaired lipid profiles. The goal is optimal E2 — typically 20–40 pg/mL on cycle.
Anastrozole: Reversible Competitive Inhibitor
Anastrozole (Arimidex) binds to aromatase and suppresses it reversibly — stop taking it and enzyme activity returns. It reduces estradiol by 85–90% in clinical use. On-cycle dose: 0.25–1 mg every other day (EOD), adjusted based on E2 bloodwork. Starting dose recommendation: 0.5 mg EOD from week 2. Advantages: well-studied, easy to titrate, immediately reversible if E2 crashes. Disadvantage: can negatively impact lipid profile (reduces HDL) and reduces bone mineral density with long-term use.
Exemestane: Steroidal Suicide Inhibitor
Exemestane (Aromasin) is a steroidal aromatase inactivator — it permanently binds to and destroys the aromatase enzyme. New enzyme must be synthesised, so effects last 72+ hours per dose. On-cycle dose: 12.5–25 mg every other day. Advantages over Anastrozole: it does not negatively affect lipid profiles (in fact, some studies show mild HDL preservation). It has mild intrinsic androgenic activity (it is a steroidal compound), providing slight anabolic benefit. It is preferred during PCT (Clomid-based) because it does not interfere with SERM activity as strongly as Anastrozole.
Head-to-Head: Which to Choose?
For most cycles: Anastrozole is simpler to manage due to its reversible action. Ideal for beginners and short cycles. For PCT or long cycles: Exemestane is preferred — its steroidal structure is more compatible with the recovering hormonal environment, and its lipid-neutral profile matters more during the extended period of PCT. If running SERM-based PCT: Exemestane + Nolvadex is the classic combination (avoiding the Anastrozole-Nolvadex interaction where anastrozole can reduce tamoxifen efficacy at the hypothalamic level).
Recognising and Correcting Low Estrogen
Over-suppression signs: painful joints (especially elbows and knees), sudden libido loss, extreme lethargy, emotional flatness, loss of morning erections. If these appear: stop AI immediately, allow E2 to recover over 5–7 days. Re-introduce at a lower dose or frequency. Regular E2 blood testing (weeks 4–6 of cycle) is the only accurate way to dial in AI dosing. Never dose AIs by feel alone. The blood test takes 15 minutes and the data it provides is invaluable for health management.
Written by
Ivan Kowalski
Exercise Physiologist