Efaproxiral sodium is a synthetic allosteric effector of hemoglobin primarily investigated in oncology as a radiosensitizer. It decreases the oxygen affinity of hemoglobin, shifting the oxygen dissociation curve to the right and facilitating oxygen unloading to hypoxic tissues, including tumors. This improves tumor oxygenation, making cancer cells more susceptible to radiation damage and enhancing the effectiveness of radiation therapy, particularly for brain metastases from breast cancer. Clinical trials have explored its use in combination with standard radiation protocols to improve treatment outcomes, though it has not received regulatory approval for clinical use. It is available for research purposes.