Letrozole is an adjuvant treatment for breast cancer; however, it can cause side effects such as bone pain, gastrointestinal reactions, liver damage, and headaches. 1. Letrozole’s effect: it has certain ovulation effect, can inhibit the conversion of androgen to estrogen, and prevent the stimulating effect of estrogen on tumor growth. It can assist in the treatment of breast cancer. 2. Side effects of Letrozole: Because Letrozole belongs to stimulating drugs, taking Letrozole can cause adverse reactions such as bone pain, and can also lead to gastrointestinal discomfort, causing nausea, headache, diarrhea, constipation and other adverse reactions. In addition, it can cause itching, rash, joint pain, fatigue, insomnia, dizziness, edema, hypertension, cardiac arrhythmia, thrombosis, vaginal bleeding, chest pain, dyspnea and cough. And when taken, it can irritate the liver and cause liver damage. Patients who are allergic to the active drug and/or any of the excipients. It is contraindicated in premenopausal endocrine states; pregnant and lactating women. The drug needs to be taken under medical supervision.