Here, we performed a retrospective cohort study using data from the Neo‐LaTH / JBCRG‐16 randomized phase II clinical trial of primary HER2 positive breast cancer patients treated with neoadjuvant lapatinib to investigate if the timing of lapatinib administration is associated with its toxicity, pharmacokinetic stability, dose modification, and therapeutic efficacy. This evidence concerns the gene ERBB2 and breast carcinoma.