Are there people who should not take femara (letrozole) for breast cancer?

Yes: Femara (letrozole) is only effective for hormone-sensitive (estrogen receptor +) breast ca patients. Drugs like femara, called aromatase inhibitors, act by blocking the conversion of aromatase to estrogen in the body; it has no effect on estrogen production by the ovaries. Therefore, these drugs are not effective in pre-menopausal women.

Premenopausal women: This is an aromatase inhibitor. This class of drugs is not effective for premenopausal women. It is used in the adjuvant setting and occasionally for metastasis.