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

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.
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.