Drug Interaction Report

Lapatinib may increase the blood levels of fostamatinib. In some cases, this may increase the risk and/or severity of side effects such as nausea, diarrhea, high blood pressure, liver problems, and low white blood cell counts. You may also be more likely to develop infections as a result of low white blood cell counts. You should seek immediate medical attention if you have signs and symptoms of infection such as fever, chills, diarrhea, sore throat, muscle aches, shortness of breath, blood in phlegm, weight loss, red or inflamed skin, body sores, and pain or burning during urination. Talk to your doctor if you have any questions or concerns. Your doctor may be able to prescribe alternatives that do not interact, or you may need a dose adjustment or more frequent monitoring to safely use both medications. It is important to tell your doctor about all other medications you use, including vitamins and herbs. Do not stop using any medications without first talking to your doctor.

Take lapatinib on an empty stomach 1 hour before or 1 hour after a meal unless otherwise directed by your doctor. This will make it easier for your body to absorb the medication. The lapatinib dose is administered once daily and should not be divided. The usual dose of lapatinib is equal to 5 tablets. You may swallow each tablet one at a time, but take the entire lapatinib dose at the same time each day. If you are receiving therapy with lapatinib you should be advised to avoid grapefruits and grapefruit juice. Grapefruit can raise the levels of lapatinib in your body and lead to dangerous side effects.