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.