Understanding Number Needed to Treat (NNT) for Breast Cancer Prevention
Arimidex (anastrozole) is more effective than tamoxifen at preventing breast cancer in high-risk postmenopausal women, requiring treatment of only 29 women for 5 years to prevent one case of breast cancer, compared to 49 women for tamoxifen. 1
What Does "Number Needed to Treat" Mean?
Think of it like this: Imagine you have a big group of women who are at higher risk for getting breast cancer. If you give all of them a medicine to help prevent breast cancer, not every single woman will benefit—some would have stayed healthy anyway, and some might still get breast cancer despite the medicine.
The "number needed to treat" tells us how many women need to take the medicine for 5 years so that ONE woman avoids getting breast cancer who would have gotten it without the medicine. 1
Breaking Down the Numbers
For Arimidex (Anastrozole):
- 29 women need to take the medicine for 5 years 1
- Out of those 29 women, 1 woman will avoid getting breast cancer because of the medicine
- The other 28 women either wouldn't have gotten breast cancer anyway, or the medicine didn't prevent it for them
- This means Arimidex reduces breast cancer risk by about 53% in postmenopausal women at increased risk 1
For Tamoxifen:
- 49 women need to take the medicine for 5 years 1
- Out of those 49 women, 1 woman will avoid getting breast cancer because of the medicine
- The other 48 women either wouldn't have gotten breast cancer anyway, or the medicine didn't prevent it for them
- This means tamoxifen reduces breast cancer risk by about 49% in women at increased risk 1
Why This Matters
The lower the number, the better the medicine works. Since 29 is smaller than 49, Arimidex is more effective—you need to treat fewer women to prevent one case of breast cancer. 1
However, both medicines work quite well, and the choice between them depends on other factors like side effects and whether a woman has gone through menopause yet. 1 Arimidex only works in women who have already gone through menopause (postmenopausal women), while tamoxifen can work in women before or after menopause. 1
Important Context
These numbers apply specifically to postmenopausal women who are at higher-than-average risk for breast cancer—not all women. 1 Women at higher risk include those with:
- Atypical hyperplasia (abnormal breast cells) 1
- Lobular carcinoma in situ (LCIS) 1
- A 5-year breast cancer risk of 1.66% or higher based on risk calculators 1
- Strong family history of breast cancer 1
For women with atypical hyperplasia specifically, the benefit is even more dramatic—tamoxifen reduces their risk by 86%, meaning even fewer women need to be treated to prevent one case. 1