Presented herein are methods for preventing or treating tumor growth, tumor metastasis and/or abnormal proliferation of tumor cells in a subject, wherein the methods involve administration of a pharmaceutical composition comprising methylnaltrexone. Also presented herein are methods for inhibiting or slowing the growth of a tumor in a subject, wherein the methods include selecting a subject who is a suitable candidate for treatment with methylnaltrexone, and administering a composition comprising methylnaltrexone to the subject.