Letrozole tablets are used to treat some forms of breast cancer in women after menopause, such as hormone-receptor-positive breast cancer. Letrozole is also used to help prevent cancer from returning. A natural hormone called estrogen stimulates the growth of certain breast tumors. Letrozole reduces the amount of estrogen produced by the body, which helps to halt or reverse the progression of certain breast tumors. Common side effects can be like include hot flashes, difficulty sleeping, tiredness, and low mood.