Combination therapy with oleocanthal (at concentrations of 12–15 μM) and lapatinib (at doses of 30–60 nM) resulted in synergistic antiproliferative effects against the HER2 (Human epidermal growth factor receptor 2)-positive BT-474 and SK-BR-3 human breast cancer cell lines and significantly inhibited EGFR (Epidermal growth factor receptor), HER2 and c-Met receptor activation, as compared to oleocanthal monotherapy or lapatinib monotherapy. The gene discussed is ERBB2; the disease is breast carcinoma.