The present invention relates to a method for preventing cancer by using a phthalide compound, wherein the phthalide compound has an effect of increasing the oxygen release efficiency of hemoglobin (Hb) of a subject to increase the oxygenation level of organs and tissue cells, thereby preventing the cellular oxygenation level against falling below the critical cellular oxygen requirements under which the normal cells can turn cancerous. Although cancer may be caused by a variety of reasons, including congenital inheritance, external environment, air pollution or poor living and dietary habits, there is only one primary and common reason in causing cancers, the excessively low cellular oxygenation level. When the oxygenation level of any cell falls below 60% of its physiological oxygen requirements, the normal cell may turn cancerous. The present invention is to use the phthalide compound to substitute for or cooperate with 2,3-BPG of a subject to lower the oxygen affinity of hemoglobin (Hb) and to facilitate the release of oxygen from hemoglobin to tissue cells, as a result, the cellular oxygenation level is maintained always higher than the critical threshold for the normal cells to turn cancerous, and by doing so to prevent the normal cells from turning cancerous and prevent the developments of cancers.
