SIDE EFFECTS: See also Warning section. Nausea, vomiting, upset stomach, mouth sores, mild rash, dry skin, and trouble sleeping may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly. Diarrhea is a common side effect and may become severe. Tell your doctor right away if diarrhea occurs or if you have signs of a severe loss of body water (dehydration) such as dizziness or decreased urination. Your doctor should prescribe additional medication to help control your symptoms. If your diarrhea becomes severe, your doctor may need to stop or delay your lapatinib treatment. Treatment with this drug may sometimes cause your hands/feet to develop a skin reaction called hand-foot syndrome (palmar-plantar erythrodysesthesia). Tell your doctor right away if you experience swelling, pain, redness, peeling, blisters, or tingling/burning of the hands/feet. The symptoms can be made worse by heat/pressure on your hands/feet. Avoid prolonged sun exposure, tanning booths, and sunlamps, as well as unnecessary exposure to heat (e.g., hot dishwater, long hot baths). Use a sunscreen and wear protective clothing when outdoors. Avoid pressure on elbows, knees, and soles of feet (e.g., leaning on elbows, kneeling, taking long walks). Wear loose clothing and comfortable shoes. Depending on how severe your hand-foot syndrome is, your doctor may give you an additional medication to reduce the symptoms, or stop or delay your lapatinib treatment. Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Tell your doctor right away if you have any serious side effects, including: unusual tiredness, shortness of breath, swelling ankles/feet. Get medical help right away if you have any very serious side effects, including: fast/pounding/irregular heartbeat, dizziness, fainting. A very serious allergic reaction to this drug 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 lapatinib, tell your doctor or pharmacist if you are allergic to it; 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 this medication, tell your doctor or pharmacist your medical history, especially of: heart disease (e.g., irregular heartbeat), liver disease. Lapatinib 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 lapatinib, 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 lapatinib safely. Older adults may be more sensitive to the side effects of this drug, especially QT prolongation (see above). This medication is not recommended for use during pregnancy. It may harm an unborn baby. Therefore, it is important to prevent pregnancy while taking this medication. Consult your doctor for more details and to discuss the use of reliable forms of birth control (e.g., condoms, birth control pills) while taking this medication. If you become pregnant or think you may be pregnant, tell your doctor right away. It is not known whether this drug passes into breast milk. Because of the possible risk to the infant, breast-feeding while using this drug is not recommended. Consult your doctor before breast-feeding.

DRUG INTERACTIONS: 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. This medication can slow down the removal of other medications from your body, which may affect how they work. Examples of affected drugs include certain "statin" cholesterol medications (simvastatin, lovastatin, atorvastatin), tacrolimus, trazodone, digoxin, among others. Other medications can affect the removal of lapatinib from your body, which may affect how lapatinib works. Examples include dexamethasone, cimetidine, St. John's wort, azole antifungals (such as itraconazole, ketoconazole), macrolide antibiotics (such as erythromycin, clarithromycin), HIV protease inhibitors (such as ritonavir, saquinavir), rifamycins (such as rifabutin), certain anti-seizure medicines (such as phenytoin), among others. Many drugs besides lapatinib may affect the heart rhythm (QT prolongation), including dofetilide, pimozide, procainamide, amiodarone, quinidine, sotalol, macrolide antibiotics (such as erythromycin), among others. Therefore, before using lapatinib, report all medications you are currently using to your doctor or pharmacist.

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. Symptoms of overdose may include: severe diarrhea/vomiting.