The purpose of this study is to evaluate the photostabilization mechanism of risperidone tablets. Risperidone is widely used for sensory integration disorder. It is formulated as tablets, orally disintegrating tablets, fine granules, oral solutions, and intramuscular injections. We found that risperidone was unstable in tablets and generated oxidized products. Formation of the oxidized product R5 was promoted in the presence of hydroxypropylcellulose by photo irradiation. On the other hand, photo stability improved greatly when carmellose (CMC) or carmellose calcium (CMC-Ca) was used as a disintegrant. Since CMC and CMC-Ca are acidic substances, the photo stability of tablets may have been affected by pH. Therefore, the effects of different pHs were examined. Risperidone was dissolved in methanol and the buffer (in the presence or absence of hydroxypropylcellulose) at different pHs (1.2,

3.0, 4.0, 5.0, and 6.8) was added. The photo degradation of risperidone was not observed at less than pH 3.0 in the presence of hydroxypropylcellulose, and low pHs improved the photostability of risperidone. On the other hand, risperidone solution without hydroxypropylcellulose was stable at all pH values. Therefore, risperidone was photochemically oxidized in the presence of hydroxypropylcellulose at high pHs.