Lapatinib belongs to the class of medications called antineoplastics (anticancer medications). It is used in combination with another anticancer medication, capecitabine, to treat advanced or metastatic (cancer that has spread to other parts of the body) breast cancer that is HER2-receptor positive and has progressed despite treatment with other anticancer medications, including trastuzumab. Lapatinib is also used in combination with the medication letrozole, when suitable, for the treatment of metastatic post-menopausal breast cancer in women whose cancer is HER2-receptor positive.

This medication delays the progression of breast cancer but does not reduce its symptoms or improve survival. It works by interfering with the growth of certain cancer cells. Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. As well, some forms of this medication may not be used for all of the conditions discussed here. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

Lapatinib/capecitabine combination: The usual dose of lapatinib is 1,250 mg (5 tablets) once daily. It should be taken at least 1 hour before or at least 1 hour after a low-fat meal. The tablets should be swallowed whole with water. Do not drink grapefruit juice while you are taking this medication.

Lapatinib/letrozole combination: The usual dose of lapatinib is 1,500 mg (6 tablets) once daily. It should be taken at least 1 hour before or at least 1 hour after a low-fat meal. The tablets should be swallowed whole with water. Do not drink grapefruit juice while you are taking this medication.

Many things can affect the dose of a medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor. If you miss a dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Store this medication at room temperature (but not above 30°C) and keep it out of the reach of children.

Each yellow, oval, biconvex, film-coated tablet, with one side plain and the opposite side debossed with "GS XJG", contains lapatinib ditosylate 250 mg. Nonmedicinal ingredients: tablet core: magnesium stearate, microcrystalline cellulose, povidone, and sodium starch glycolate; coating: hypromellose, iron oxide red, iron oxide yellow, macrogol/PEG 400, polysorbate 80, and titanium dioxide.

Do not take this medication if you are allergic to lapatinib or any ingredients of the medication or the container.

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• back pain
• constipation
• deep cracks in the skin
• difficulty sleeping
• dry skin
• fever
• hair loss or thinning
• headache
• indigestion
• joint or bone pain
• loss of appetite
• mouth sores or sore mouth
• nail tenderness, infection, or swelling
• nausea
• nosebleeds
• pain in the arms or legs
• skin rash
• stomach pain
• sun sensitivity
• tingling, numbness, pain, swelling, or redness of the palms of the hands or soles of the feet
• tiredness
• vomiting

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• diarrhea
• difficulty breathing
• swelling in the arms or legs
• symptoms of heart problems (e.g., shortness of breath, swelling of the ankles or feet, chest pain, irregular heartbeat)
• symptoms of irregular heartbeat (e.g., chest pain, dizziness, rapid, pounding heartbeat, shortness of breath)
• symptoms of liver problems (e.g., nausea, vomiting, abdominal pain, yellowing of the skin and eyes, itchy skin, dark urine)
• symptoms of lung problems (e.g., shortness of breath or difficulty breathing, chest pain, cough)

Stop taking the medication and seek immediate medical attention if any of the following occur:

• symptoms of a severe allergic reaction (e.g., hives; difficulty breathing; swelling of the face, mouth, tongue, or throat)
• signs of a severe skin reaction such as blistering, peeling, a rash covering a large area of the body, a rash that spreads quickly, or a rash combined with fever or discomfort

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Diarrhea: This medication can cause diarrhea that can lead to dehydration. Dehydration can cause many symptoms, including decreased urine production, dry and sticky mouth, sleepiness, dizziness, headache, thirst, and confusion. In some cases, dehydration can become a medical emergency. If you have diarrhea or any of these symptoms while taking this medication, let your doctor know.

Heart effects: This medication can affect heart function. Your doctor will monitor you for these effects while you are taking this medication. If heart problems occur, your doctor may stop this medication and restart it if your heart function improves.

Lapatinib can also have an effect on the electrical activity of the heart called QT prolongation. If you experience an abnormal heartbeat, dizziness, palpitations, seizures, or fainting while taking this medication, get immediate medical attention.

If you are at risk of complications due to QT prolongation (e.g., seniors, women, heart disease, abnormal heart rhythms, stroke, eating disorders, diabetes, a family history of sudden cardiac death) or are taking other medications that can cause QT prolongation (see the section "What medications can interact with this medication?"), discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Liver problems: This medication can cause reduced liver function and can cause liver failure, which can be fatal. Your doctor will monitor your liver function with blood tests while you are taking this medication. If your liver function worsens, your doctor may stop this medication.

If you have liver problems, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

If you experience symptoms of liver problems such as fatigue, feeling unwell, loss of appetite, nausea, yellowing of the skin or whites of the eyes, dark urine, pale stools, abdominal pain or swelling, and itchy skin, contact your doctor immediately.

Lung effects: This medication can cause lung problems. Your doctor will monitor you for these effects while you are taking this medication. If you experience shortness of breath or difficulty breathing while taking this medication, contact your doctor immediately. If lung problems occur, your doctor may need to stop this medication.

Skin reactions: This medication may increase the sensitivity of the skin to sunlight, increasing the risk of sunburn. Avoid exposure to sunlight for long periods of time, particularly between the hours of 10 am and 2 pm, and wear a broad-spectrum sunscreen and lip balm with an SPF of 30 or greater.

Rarely, people taking lapatinib may experience a severe skin reaction that can be life-threatening. These reactions may start as a skin rash that blisters, sores on the lips, eyes, or covers a large area of the body. It can develop into an exfoliating skin condition, with peeling, shedding, or scaling of the skin. If you experience any of these symptoms, or any other unusual skin reaction of the skin, contact your doctor immediately.

Pregnancy: This medication should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while taking this medication, contact your doctor immediately. People who may become pregnant should use contraception and avoid becoming pregnant while they are taking this medication and for at least 5 days after the last dose.

Breast-feeding: It is not known if lapatinib passes into breast milk. If you are breast-feeding and are taking this medication, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding. Breast-feeding while taking this medication is not recommended.

Children: The safety and effectiveness of using this medication have not been established for children.

There may be an interaction between lapatinib and any of the following:

• aliskiren
• alprazolam
• antiarrhythmics (e.g., amiodarone, dronedarone, procainamide)
• other anticancer medications (e.g., doxorubicin, teniposide, topotecan, venetoclax, vincristine)
• antipsychotics (e.g., chlorpromazine, haloperidol, risperidone)
• apalutamide
• aprepitant
• "azole" antifungals (e.g., ketoconazole, itraconazole, voriconazole)
• bilastine
• bosentan
• carbamazepine
• carvedilol
• cetirizine
• cobicistat
• colchicine
• conivaptan
• cyclosporine
• dabigatran
• dexamethasone
• digoxin
• diltiazem
• disopyramide
• edoxaban
• efavirenz
• elagolix
• eliglustat
• enzalutamide
• etravirine
• everolimus
• finerenone
• glecaprevir
• grapefruit juice
• HIV protease inhibitors (e.g., atazanavir, darunavir, ritonavir)
• lemborexant
• letermovir
• lomitapide
• lumacaftor and ivacaftor
• macrolide antibiotics (e.g., azithromycin, clarithromycin, erythromycin)
• methadone
• mifepristone
• modafinil
• morphine
• nadolol
• naloxegol
• nirmatrelvir
• phenobarbital
• phenytoin
• pibrentasvir
• primidone
• other protein kinase inhibitors (e.g., ceritinib, dabrafenib, idelalisib, imatinib, pazopanib)
• repaglinide
• rifabutin
• rifampin
• rifaximin
• romidepsin
• St. John's wort
• silodosin
• sirolimus
• "statin" anti-cholesterol medications (e.g., rosuvastatin, simvastatin)
• tacrolimus
• tenofovir
• triazolam
• verapamil

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.