Short answer: Both are aromatase inhibitors for breast cancer in postmenopausal women.12 Femara (letrozole) 2.5 mg once a day can follow surgery directly, follow 5 years of tamoxifen, or treat advanced cancer. Aromasin (exemestane) 25 mg once daily after a meal follows 2 to 3 years of tamoxifen to complete 5 years, or treats advanced cancer after tamoxifen. Neither label reports a trial against the other.
| Femara | Aromasin | |
|---|---|---|
| Drug | Letrozole1 | Exemestane2 |
| Early breast cancer | After surgery, or as extended treatment after 5 years of tamoxifen1 | After 2 to 3 years of tamoxifen, switching to complete 5 years in total2 |
| Advanced breast cancer | First- or second-line treatment1 | After the cancer has progressed on tamoxifen2 |
| Dose | 2.5 mg once a day with or without food; every other day with cirrhosis or severe liver impairment1 | 25 mg once daily after a meal; 50 mg with a strong CYP3A4 inducer2 |
| Trial against tamoxifen | BIG 1-98: high cholesterol 52.3% vs 28.6%, hot flashes 33.5% vs 38.0%, joint pain 25.4% vs 20.6%1 | IES: hot flushes 21% vs 20%, fatigue 16% vs 15%, joint pain 15% vs 9%2 |
| Spine bone density vs tamoxifen | Median fall of 4.1% in 2 years vs a 0.3% rise1 | -3.1% at 24 months vs -0.2%2 |
| Missed dose | No instruction in the label1 | No instruction in the label2 |
| Boxed warning | None1 | None2 |
Both labels warn about bone loss, with bone density monitoring, and harm to an unborn baby: effective birth control for at least 3 weeks after the last Femara dose, and for 1 month after the last Aromasin dose.12 Femara adds raised cholesterol, tiredness and dizziness, and a lower dose with cirrhosis or severe liver impairment.1 Aromasin adds a vitamin D check before starting, no estrogen-containing medicines alongside it, and that it is not for women before menopause.2 These numbers come from separate trials with different people, lengths and methods, so they are not a head-to-head comparison.
Which, if either, fits is for your prescriber. More detail: Femara side effects, Aromasin side effects, the Femara and Aromasin dosing schedules, the Femara missed-dose page, Anastrozole vs letrozole: what is the difference? and Anastrozole vs exemestane: how do these aromatase inhibitors differ?.