• If you are allergic to any medicines, dyes, additives, or foods.

• If you have any history of liver disease (including hepatitis). This drug is cleared from the body mainly by the liver. Reduced liver function may result in more drug than expected staying in the body, which could lead to unwanted side effects. Your doctor may need to adjust your dose accordingly.

• If you have a history of congestive heart failure or other heart problems, including problems with the heart rhythm. This drug may affect the heart rhythm or how strongly the heart beats.

• If you have any other medical conditions such as kidney disease, high blood pressure, diabetes, gout, or infections. These conditions may require that your medicine dose, regimen, or timing be changed.

• If you are pregnant, trying to get pregnant, or if there is any chance of pregnancy. This drug can cause problems with the fetus if taken at the time of conception or during pregnancy. It is important to check with your doctor about whether birth control should be used and what kinds of birth control can be used with this medicine. In pregnant women, treatment with this drug should be used only if the potential benefit to the mother outweighs the risk to the fetus.

• If you are breast-feeding. While no studies have been done, this drug may pass into breast milk and affect the baby. Breast-feeding is not recommended during treatment with this drug.

• If you think you might want to have children in the future. It is not known whether this drug can affect fertility. Talk with your doctor about the possible risk with this drug and the options that may preserve your ability to have children.

• About any other prescription or over-the-counter medicines you are taking, including vitamins and herbs.

Check with your doctor, nurse, or pharmacist about any other medicines, herbs, and supplements you are taking, and whether alcohol can cause problems with this medicine.

Keep the medicine in a tightly closed container away from heat and moisture and out of the reach of children and pets.

This drug may affect the rhythm or function of the heart. Tell your doctor if you have any history of heart problems before starting treatment. He or she may order tests of the heart before treatment begins. Possible symptoms of heart problems might include palpitations, chest pain, increased coughing, trouble breathing (especially at night), or swelling in the ankles or legs. Tell your doctor right away if you start to notice any of these symptoms once treatment beings.

This drug may affect liver function, which in rare cases may be serious or even life threatening. Your doctor will likely do blood tests to check your liver function before and during treatment. Be sure to tell your doctor if you notice skin itching, yellow eyes or skin, dark urine, or pain or discomfort in the right upper area of the belly, which could be signs of liver problems.

This drug may cause diarrhea. If left unchecked, this could lead to dehydration and chemical imbalances in the body. Your doctor may prescribe medicine to help prevent or control this side effect. Your doctor may need to change your dose of lapatinib or stop it for a time if the problem is severe.

A rash on the face, neck, and trunk is a common effect, beginning during the first 2 weeks of treatment and going away once treatment is stopped. The rash may become severe in rare cases. Tell your doctor if it becomes bothersome. You should wear sunscreen and hats and limit your sun exposure while receiving this drug, as sunlight may make the rash worse.

This drug can cause a condition known as hand-foot syndrome, in which a person may experience pain, numbness, tingling, reddening, or swelling in the hands or feet. Peeling, blistering, or sores on the skin in these areas are also possible. Let your doctor know right away if you notice any of these symptoms.

Your doctor will likely test your blood throughout your treatment, looking for possible effects of the drug on blood counts or on blood chemistry levels. Based on the test results, you may be given medicines to help treat any effects. Your doctor may also need to reduce or delay your next dose of this drug, or even stop it altogether.

This drug may lower your red blood cell count. If this occurs, it is usually a few months after starting treatment. A low red blood cell count (known as anemia) can cause shortness of breath, or make you feel weak or tired all the time. Your doctor may give you medicines to help prevent or treat this condition.

• diarrhea*
• rash*
• hand-foot syndrome*

• nausea
• vomiting
• feeling tired
• mouth sores
• loss of appetite
• heartburn

• effects on heart rhythm or function*
• low red blood cell count (anemia)*
• liver problems, which might show up as yellow eyes or skin, skin itching, dark urine, pain in the right side of the belly, or abnormal blood tests*

*See "Precautions" section for more detailed information.