Short answer: The labels say they can. Arimidex (anastrozole) and Femara (letrozole) lower estrogen, and both labels report falls in bone density and more fractures than on tamoxifen. In a Femara bone study, spine density fell a median 4.1% over 2 years vs a 0.3% rise on tamoxifen. Both labels suggest bone density monitoring, and Prolia is approved for women on these medicines who are at high risk of fracture.
Aromatase inhibitors block the enzyme that makes estrogen in postmenopausal women, which is how they treat hormone receptor-positive breast cancer. Estrogen also helps keep bone. The Arimidex label says that because it lowers circulating estrogen, it may reduce bone mineral density, which may increase the risk of fractures. Tamoxifen works differently, and in the trials below it raised spine and hip density.
| Trial | Comparison | Result |
|---|---|---|
| Femara bone study (263 women) | Letrozole vs tamoxifen, 2 years | Spine density down a median 4.1% vs up 0.3% |
| BIG 1-98 | Letrozole vs tamoxifen, median 96 months | Fractures 14.7% vs 11.4%; osteoporosis 5.1% vs 2.7% |
| MA-17 (after 5 years of tamoxifen) | Letrozole vs placebo, median 62 months | Fractures 13.3% vs 7.8%; new osteoporosis 14.5% vs 7.8% |
| MA-17B substudy | Letrozole vs placebo, 2 years | Hip density down a median 3.8% vs 2.0%; spine change not significantly different |
| ATAC | Anastrozole vs tamoxifen | Fractures during treatment 10% vs 7% (spine, hip and wrist); first fractures during or after treatment 15% vs 11% |
The Arimidex label adds that the higher fracture rate during treatment did not continue after treatment stopped. In the Femara bone study, no woman with normal density at the start became osteoporotic over 2 years.
Both labels say to consider monitoring bone mineral density, and the Arimidex patient information says a bone density test may be ordered before and during treatment. The Arimidex label describes a trial in which all women took calcium and vitamin D: those not given a bisphosphonate had small falls in spine density at 12 months, while the bisphosphonate risedronate preserved bone density in most women at risk of fracture. It says bone health should be managed under the guidelines used for other postmenopausal women at similar fracture risk.
The Prolia (denosumab) label includes an approval to increase bone mass in women on adjuvant aromatase inhibitor therapy for breast cancer who are at high risk of fracture. Which bone medicine, if any, is right is a decision for the oncology team and prescriber. See calcium and vitamin D with bone medicines for what those labels say.
Some men on testosterone therapy are prescribed anastrozole to control estradiol. The numbers above come from postmenopausal women with breast cancer at full cancer doses, so they do not show what lower doses do in men. That use is covered on the estrogen on TRT page. Dose Track can log either medicine with bone density scan dates, so results can be read against how long it has been taken.
Read more on Dose Track: Aromatase inhibitor · Bone mineral density · Arimidex side effects · Femara side effects · Anastrozole vs letrozole