Short answer: Both treat hormone receptor-positive breast cancer.12 Arimidex, an aromatase inhibitor, is approved only for postmenopausal women; tamoxifen, an antiestrogen, also covers metastatic disease in men, DCIS and risk reduction in high-risk women. In ATAC, reported in the Arimidex label, fractures and joint symptoms were more common on Arimidex, and hot flashes, vaginal bleeding and blood clots on tamoxifen. Only tamoxifen has a boxed warning.
| Arimidex | Tamoxifen | |
|---|---|---|
| Approved for | Postmenopausal women: after surgery for hormone receptor-positive early breast cancer, first treatment of advanced breast cancer, and advanced disease 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 | 1 mg once a day, with or without food1 | 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 |
| How long | After surgery: best length unknown, 5 years in ATAC; advanced cancer: until it progresses1 | Trial data support 5 years after surgery2 |
| ATAC: hot flashes | 36%1 | 41%1 |
| ATAC: joint symptoms | 36%1 | 29%1 |
| ATAC: all fractures | 10%1 | 7%1 |
| ATAC: vaginal bleeding / discharge | 5% / 4%1 | 10% / 13%1 |
| ATAC: venous blood clots / endometrial cancer | 3% / 0.2%1 | 5% / 0.6%1 |
| Boxed warning | None1 | Uterine cancers, stroke and pulmonary embolism in women using it after DCIS or to lower breast cancer risk2 |
ATAC compared Arimidex 1 mg (3,092 women) with tamoxifen 20 mg (3,094 women) after surgery for early breast cancer, with a median of about 60 months of treatment.1 Beyond the table above, raised cholesterol was reported in 9% on Arimidex vs 3.5% on tamoxifen, and carpal tunnel syndrome in 2.5% vs 0.7%. Spine and hip bone density fell on average on Arimidex and rose on tamoxifen. In women who already had ischemic heart disease, heart events from reduced blood flow were 17% vs 10%. Serious adverse reactions overall were similar, 50% vs 51%.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 Its other warnings cover eye changes, liver effects, blood counts and blood thinners such as warfarin. The Arimidex label warns about heart events in women with existing ischemic heart disease, bone loss, cholesterol and harm to an unborn baby.1
Both labels say the two should not be taken together; in ATAC the combination arm showed no benefit over tamoxifen alone.12 Which, if either, fits is for your prescriber. More detail: Arimidex side effects, tamoxifen side effects, the Arimidex and tamoxifen dosing schedules, the Arimidex and tamoxifen missed-dose pages, and Anastrozole vs letrozole: what is the difference?.