PURPOSE: To obtain a drug useful for preventing and treating diseases caused by cell disorder by active oxygen, consisting essentially of 9-cis-octadecenoic acid or 9-trans-octadecenoic acid.

CONSTITUTION: 9-cis-Octadecenoic acid or 9-trans-octadecenoic acid as a main component is blended with pharmaceutically acceptable carrier, excipient, diluent, etc., to give a drug to be administered orally or parenterally as tablet, capsule, solution, suppository, injection, etc. A dose of the active ingredient compound is 5-50mg/kg weight/time and administered once-three times daily in the case of oral administration, 5-20mg/kg weight/time as suppository and 0.1-20mg/kg weight/time as injection and administered once-twice daily, respectively. The compound has extremely low toxicity and excellent tissue transition because the compound is smoothly taken in biomembrane.