A drug used to treat HER2 positive (HER2+) breast cancer that is advanced or has spread to other parts of the body. It is used with capecitabine in patients whose cancer has not gotten better with other anticancer drugs. It is used with letrozole in postmenopausal women whose cancer is hormone receptor positive (HR+) and who need hormone therapy. It is also being studied in the treatment of other types of cancer. Lapatinib ditosylate blocks epidermal growth factor receptor (EGFR) and other proteins, which may help keep cancer cells from growing. Lapatinib ditosylate is a type of ErbB2 and EGFR dual tyrosine kinase inhibitor. Also called GW572016, lapatinib, and Tykerb.