Aromatase inhibitors (AI)Aromatase inhibitor (AI)A compound that blocks the aromatase enzyme, reducing the conversion of testosterone into estrogen (E2).
Anastrozol vs Letrozol vs Exemestane
The table compares the compounds’ key data from their entry pages. Open each compound’s full entry for details. Educational content, not medical advice.
 Anastrozole (Arimidex) | Letrozole (Femara) | Exemestane (Aromasin) | |
|---|---|---|---|
| What it is | Triazole-class non-steroidal aromatase inhibitor (AI). FDA-approved (1995 AstraZeneca) for postmenopausal ER+ breast cancer. AAS-cycle E2-control golden-standard, 0.25-1 mg EOD-2x/week titrated by bloodwork. | Triazole-class non-steroidal AI, stronger than anastrozole (~99% E2 suppression at 2.5 mg/day). FDA-approved (1997 Novartis). AAS-niche: hard-suppression Tren+Mast E2-control + gynecomastia reversal (2.5 mg/day × 2-3 weeks). | Steroidal "suicide" (irreversible) aromatase inhibitor. FDA-approved (1999 Pfizer). Unique: 17-hydro-exemestane metabolite is mildly androgenic – SHBG drop + visible free-Test rise. In AAS used when E2 stability or SHBG emphasis matters. |
| MechanismMechanism of actionThe molecular pathway by which a compound acts in the body (e.g. which receptor or enzyme it binds). | Reversible non-steroidal CYP19 aromatase inhibitor, ~97% serum E2 suppression at 1 mg/day | Reversible non-steroidal CYP19 aromatase inhibitor, ~99% serum E2 suppression at 2.5 mg/day | Steroidal "suicide" (irreversible covalent) CYP19 aromatase inhibitor – a single dose permanently inactivates a portion of the pool |
| Dosing (AAS) | 0.25-1 mg EOD-2x/week titrated by bloodwork (E2 trough 20-30 pg/mL LC-MS/MS) | 0.25-1.25 mg EOD bloodwork-titrated; gyno reversal 2.5 mg/day × 14-21 days | 12.5-25 mg EOD or 25 mg/day |
| Half-lifeHalf-lifeThe time for the compound’s plasma concentration to fall by half; indicates how fast it clears and how often it is dosed. | ~50 hours (single daily dose, steady-state day 7) | ~42 hours | ~27 h (parent), ~24-48 h active metabolite; pharmacodynamic effect 5-7 days |
| OnsetOnsetThe time from dosing until a measurable effect appears. | E2 reduction measurable 24-48 h, full effect 5-7 days | E2 reduction measurable 24-48 h, plateau 4-6 days | E2 reduction measurable 12-24 h, full effect 3-5 days |
| Legal statusLegal / WADA statusThe compound’s regulatory status (e.g. prescription) and its place on the WADA prohibited-in-sport list. | FDA + EMA Rx, HU + PL approved, WADA S4.1 banned | FDA + EMA Rx, HU + PL approved, WADA S4.1 banned | FDA + EMA Rx, HU + PL approved, WADA S4.1 banned |
| Full entry | Open → | Open → | Open → |
What it is
Anastrozole (Arimidex)Triazole-class non-steroidal aromatase inhibitor (AI). FDA-approved (1995 AstraZeneca) for postmenopausal ER+ breast cancer. AAS-cycle E2-control golden-standard, 0.25-1 mg EOD-2x/week titrated by bloodwork.
Letrozole (Femara)Triazole-class non-steroidal AI, stronger than anastrozole (~99% E2 suppression at 2.5 mg/day). FDA-approved (1997 Novartis). AAS-niche: hard-suppression Tren+Mast E2-control + gynecomastia reversal (2.5 mg/day × 2-3 weeks).
Exemestane (Aromasin)Steroidal "suicide" (irreversible) aromatase inhibitor. FDA-approved (1999 Pfizer). Unique: 17-hydro-exemestane metabolite is mildly androgenic – SHBG drop + visible free-Test rise. In AAS used when E2 stability or SHBG emphasis matters.
MechanismMechanism of actionThe molecular pathway by which a compound acts in the body (e.g. which receptor or enzyme it binds).
Anastrozole (Arimidex)Reversible non-steroidal CYP19 aromatase inhibitor, ~97% serum E2 suppression at 1 mg/day
Letrozole (Femara)Reversible non-steroidal CYP19 aromatase inhibitor, ~99% serum E2 suppression at 2.5 mg/day
Exemestane (Aromasin)Steroidal "suicide" (irreversible covalent) CYP19 aromatase inhibitor – a single dose permanently inactivates a portion of the pool
Dosing (AAS)
Anastrozole (Arimidex)0.25-1 mg EOD-2x/week titrated by bloodwork (E2 trough 20-30 pg/mL LC-MS/MS)
Letrozole (Femara)0.25-1.25 mg EOD bloodwork-titrated; gyno reversal 2.5 mg/day × 14-21 days
Exemestane (Aromasin)12.5-25 mg EOD or 25 mg/day
Half-lifeHalf-lifeThe time for the compound’s plasma concentration to fall by half; indicates how fast it clears and how often it is dosed.
Anastrozole (Arimidex)~50 hours (single daily dose, steady-state day 7)
Letrozole (Femara)~42 hours
Exemestane (Aromasin)~27 h (parent), ~24-48 h active metabolite; pharmacodynamic effect 5-7 days
OnsetOnsetThe time from dosing until a measurable effect appears.
Anastrozole (Arimidex)E2 reduction measurable 24-48 h, full effect 5-7 days
Letrozole (Femara)E2 reduction measurable 24-48 h, plateau 4-6 days
Exemestane (Aromasin)E2 reduction measurable 12-24 h, full effect 3-5 days
Legal statusLegal / WADA statusThe compound’s regulatory status (e.g. prescription) and its place on the WADA prohibited-in-sport list.
Anastrozole (Arimidex)FDA + EMA Rx, HU + PL approved, WADA S4.1 banned
Letrozole (Femara)FDA + EMA Rx, HU + PL approved, WADA S4.1 banned
Exemestane (Aromasin)FDA + EMA Rx, HU + PL approved, WADA S4.1 banned