Short answer: Both are nonsteroidal aromatase inhibitors taken once a day: anastrozole 1 mg and letrozole 2.5 mg.12 Letrozole lasts a little longer (about 2 days vs 50 hours). In the FACE trial of 4,136 women with breast cancer, letrozole was not better than anastrozole.3 Neither is approved for men.
| Anastrozole (Arimidex) | Letrozole (Femara) | |
|---|---|---|
| Mechanism | Selective nonsteroidal aromatase inhibitor1 | Nonsteroidal competitive aromatase inhibitor; binds the enzyme's heme2 |
| Approval status | FDA approved for hormone receptor-positive breast cancer in postmenopausal women1 | FDA approved for hormone receptor-positive breast cancer in postmenopausal women, including extended adjuvant use2 |
| Label dosing schedule | 1 mg once daily, with or without food1 | 2.5 mg once daily, with or without food2 |
| Half-life | about 2 days (label: mean 50 hours)1 | About 2 days2 |
| Time to peak (Tmax) | about 2 hours fasted; about 5 hours after food1 | Not stated as a number; rapidly and completely absorbed2 |
| Time to steady state | About 7 days1 | 2 to 6 weeks2 |
| Estrogen lowering | About 70% in 24 hours, 80% after 14 days1 | 75% to 95% from baseline, maximal in 2 to 3 days2 |
| Taken with tamoxifen | Tamoxifen lowered anastrozole levels by 27%1 | Tamoxifen lowered letrozole levels by 37%4 |
| Notable label warnings | Bone density loss; raised cholesterol1 | Bone density loss; raised cholesterol; contraindicated in pregnancy2 |
FACE randomized 4,136 postmenopausal women with hormone receptor-positive, node-positive early breast cancer to letrozole 2.5 mg or anastrozole 1 mg daily for 5 years. Five-year disease-free survival was 84.9% on letrozole vs 82.9% on anastrozole, a difference that was not statistically significant (hazard ratio 0.93). Five-year overall survival was 89.9% vs 89.2%. The authors concluded letrozole was not superior in efficacy or safety.3
The two are chemically similar and work the same way. Letrozole has a longer half-life, about 2 days, and takes 2 to 6 weeks to reach steady levels.2 Anastrozole has a mean half-life of 50 hours and reaches steady state in about 7 days.1 Both labels warn about bone loss and raised cholesterol, and the Femara label calls for 2.5 mg every other day in people with cirrhosis and severe liver impairment.2
Use of either in men is off label. Any such use is a decision for a prescriber, with blood work.