A method and composition for preventing prostate cancer and/or reducing PSA levels and/or alleviating the symptoms of BPH (Benign Prostatic Hyperplasia) or PIN (prostatic intraepithelial neoplasia) by administration of an effective amount of a mixture of flavopereirine and alstonine. A method and composition for treating low-grade prostate cancer and preventing the onset of metastatic disease and/or reducing the doubling time of PSA levels in men with positive biopsies showing relatively low Gleason scores and morphologies characteristic of non-invasive, slow-progressing prostate cancer. The flavopereirine and alstonine can be in the form of natural extracts derived from Pao Pereira and Rauwolfia Vomitoria, respectively. Alternatively, these two active compounds can be administered in purified form. The composition can be in included in a kit along with instructions for use in a treatment regimen.