Isolated extracts of Rhodiola crenulata and their constituents sensitize tumor cells to cancer therapies, such as tamoxifen, retinoids, and radiation, and act to potentiate radiation therapy in the treatment of cancer, with or without adjuvant chemotherapy. Methods are provided for inhibiting the P13kinase/AKT pathway in cancer cells, a general survival pathway which leads to resistance of many tumors to typical therapies; decreasing cyclooxygenase (COX) activities in cancer cells, another pathway known to increase resistance to tumors; inhibiting telomerase activity in cancer cells, an activity notable for its ability to increase the life span of the cells; enhancing the ATM (Ataxia-Telangiectasia Mutated) response in cells, which is involved in DNA damage sensing and repair; modulating cellular response to oxidation, which effects the other signaling pathways on numerous levels; and altering usage of metabolic energy pathways.