SIDE EFFECTS: Tiredness, nausea, loss of appetite, vomiting, diarrhea, headache, constipation, back pain, hot flashes, unusual sweating, dizziness, or trouble sleeping may occur. Nausea and vomiting can be severe. In some cases, your doctor may prescribe medication to prevent or relieve nausea and vomiting. Eating several small meals, not eating before treatment, or limiting activity may help lessen some of these effects. If any of these effects last or get worse, tell your doctor or pharmacist promptly. Temporary hair loss may occur. Normal hair growth should return after treatment has ended. Pain or sores in the mouth and throat may occur. Brush your teeth carefully/gently, avoid using mouthwash that contains alcohol, and rinse your mouth often with cool water mixed with baking soda or salt. It may also be best to eat soft, moist foods. People using these medications may have serious side effects. However, your doctor has prescribed these drugs because he or she has judged that the benefit to you is greater than the risk of side effects. Careful monitoring by your doctor may decrease your risk. Tell your doctor right away if you have any serious side effects, including: bone fractures, mental/mood changes (such as depression, anxiety), swelling of arms/legs, signs of liver disease (such as nausea/vomiting that doesn't stop, loss of appetite, stomach/abdominal pain, yellowing eyes/skin, dark urine). Get medical help right away if you have any very serious side effects, including: fast/irregular heartbeat, severe dizziness, fainting. Letrozole may rarely cause serious (sometimes fatal) problems from blood clots (such as deep vein thrombosis, heart attack, pulmonary embolism, stroke). Get medical help right away if you have: shortness of breath/rapid breathing, chest/jaw/left arm pain, confusion, pain/swelling/warmth in the groin/calf, sudden/severe headaches, trouble speaking, weakness on one side of the body, sudden vision changes. This product may lower your ability to fight infections. This may make you more likely to get a serious (rarely fatal) infection or make any infection you have worse. Tell your doctor right away if you have any signs of infection (such as sore throat that doesn't go away, fever, chills, cough). A very serious allergic reaction to these drugs is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

PRECAUTIONS: Before taking this product, tell your doctor or pharmacist if you are allergic to ribociclib or letrozole; or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details. Before using these medications, tell your doctor or pharmacist your medical history, especially of: high cholesterol, bone problems (such as osteoporosis), heart problems (such as angina, previous heart attack), liver disease. These drugs may make you tired or dizzy. Alcohol or marijuana can make you more dizzy. Do not drive, use machinery, or do anything that needs alertness until you can do it safely. Limit alcoholic beverages. Talk to your doctor if you are using marijuana. Ribociclib may cause a condition that affects the heart rhythm (QT prolongation). QT prolongation can rarely cause serious (rarely fatal) fast/irregular heartbeat and other symptoms (such as severe dizziness, fainting) that need medical attention right away. The risk of QT prolongation may be increased if you have certain medical conditions or are taking other drugs that may cause QT prolongation. Before using ribociclib, tell your doctor or pharmacist of all the drugs you take and if you have any of the following conditions: certain heart problems (heart failure, slow heartbeat, QT prolongation in the EKG), family history of certain heart problems (QT prolongation in the EKG, sudden cardiac death). Low levels of potassium or magnesium in the blood may also increase your risk of QT prolongation. This risk may increase if you use certain drugs (such as diuretics "water pills") or if you have conditions such as severe sweating, diarrhea, or vomiting. Talk to your doctor about using ribociclib safely. Ribociclib can make you more likely to get infections or may worsen any current infections. Avoid contact with people who have infections that may spread to others (such as chickenpox, measles, flu). Consult your doctor if you have been exposed to an infection or for more details. Do not have immunizations/vaccinations without the consent of your doctor. Avoid contact with people who have recently received live vaccines (such as flu vaccine inhaled through the nose). Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products). Older adults may be more sensitive to the side effects of these drugs, especially QT prolongation (see above). Ribociclib and letrozole are used mainly by women after menopause. If you have not gone through menopause, tell your doctor if you are pregnant or plan to become pregnant. Women who may become pregnant should have a pregnancy test before starting these medications. You should not become pregnant while using ribociclib and letrozole. They may harm an unborn baby. Ask about reliable forms of birth control while taking these medications and for 3 weeks after stopping treatment. If you become pregnant, talk to your doctor right away about the risks and benefits of these medications. Since these drugs can be absorbed through the skin and lungs and may harm an unborn baby, women who are pregnant or who may become pregnant should not handle these medications or breathe the dust from the tablets. It is unknown if these drugs pass into breast milk. Because of the possible risk to the infant, breast-feeding while using these drugs and for at least 3 weeks after stopping treatment is not recommended. Consult your doctor before breast-feeding.

DRUG INTERACTIONS: See also How to Use section. Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor's approval. Some products that may interact with these drugs are: tamoxifen, estrogens (such as estradiol). Other medications can affect the removal of ribociclib from your body, which may affect how ribociclib works. Examples include rifamycins (such as rifampin, rifabutin), St. John's wort, drugs used to treat seizures (such as carbamazepine, phenytoin), among others. Ribociclib can slow down the removal of other medications from your body, which may affect how they work. Examples of affected drugs include alprazolam, eplerenone, ergot alkaloids (such as ergotamine), estazolam, flibanserin, irinotecan, ivabradine, naloxegol, ticagrelor, tolvaptan, triazolam, among others. Many drugs besides ribociclib may affect the heart rhythm (QT prolongation), such as amiodarone, chloroquine, clarithromycin, domperidone, dronedarone, haloperidol, methadone, moxifloxacin, pimozide, ondansetron, ranolazine, among others.

OVERDOSE: If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call 911. Otherwise, call a poison control center right away. US residents can call their local poison control center at 1-800-222-1222. Canada residents can call a provincial poison control center.