Short answer: Both treat hormone receptor-positive breast cancer.12 Aromasin (exemestane) is approved for postmenopausal women, after 2 to 3 years of tamoxifen to complete 5 years, or for advanced cancer after tamoxifen. Tamoxifen also covers men, DCIS and risk reduction. In IES, reported in the Aromasin label, joint pain was more common on Aromasin, and clots and uterine changes on tamoxifen. Only tamoxifen has a boxed warning.
| Aromasin | Tamoxifen | |
|---|---|---|
| Approved for | Postmenopausal women: estrogen receptor-positive early breast cancer after 2 to 3 years of tamoxifen, and advanced breast cancer that progressed after tamoxifen1 | Metastatic breast cancer in women and men, treatment after surgery, lowering invasive cancer risk after DCIS, and lowering breast cancer incidence in high-risk women2 |
| Label dose | 25 mg once daily after a meal; 50 mg with a strong CYP3A4 inducer1 | 20 to 40 mg daily for breast cancer, doses above 20 mg split morning and evening; 20 mg daily for 5 years for DCIS and risk reduction2 |
| IES: hot flushes | 21%1 | 20%1 |
| IES: joint pain | 15%1 | 9%1 |
| IES: trouble sleeping | 12%1 | 9%1 |
| IES: osteoporosis / clinical fractures | 4.6% / 4.2%1 | 2.8% / 3.1%1 |
| IES: heart events from reduced blood flow | 1.6%1 | 0.6%1 |
| Boxed warning | None1 | Uterine cancers, stroke and pulmonary embolism in women using it after DCIS or to lower breast cancer risk2 |
IES compared Aromasin 25 mg daily (2,252 women) with tamoxifen 20 mg daily (2,280 women) in postmenopausal women with early breast cancer.1 Beyond the table above, carpal tunnel syndrome was 2.4% vs 0.2%, tingling 2.6% vs 0.9% and diarrhea 4.2% vs 2.2%; tamoxifen had more muscle cramps, blood clots, endometrial hyperplasia and uterine polyps. Lumbar spine bone density changed -3.1% on Aromasin vs -0.2% on tamoxifen at 24 months. Stopping because of side effects was 6% vs 5%.1
Tamoxifen has a boxed warning for women using it after DCIS or to lower breast cancer risk: in the P-1 trial, endometrial cancer was 2.20 vs 0.71 per 1,000 women-years on placebo, stroke 1.43 vs 1.00 and pulmonary embolism 0.75 vs 0.25. The label says the benefits outweigh these risks in women already diagnosed with breast cancer.2 The Aromasin label warns about bone loss, asks for a vitamin D check before starting, says not to take it with estrogen-containing medicines, and says it is not for women before menopause.1
Which, if either, fits is for your prescriber. More detail: Aromasin side effects, tamoxifen side effects, the Aromasin and tamoxifen dosing schedules, the tamoxifen missed-dose page, Arimidex vs tamoxifen: how do the two breast cancer tablets differ? and Femara vs tamoxifen: how do the two breast cancer tablets differ?.