PURPOSE: To obtain an antiulcer agent, containing adenosine or a derivative thereof as an active ingredient, having antiulcer action substantially without any oral toxicity including an influence on enterobacteria.

CONSTITUTION: An antiulcer agent containing adenosine or a derivative thereof, preferably a compound expressed by the formula [R is H, CH3 or benzyl; R2 is H, OH, Cl, SCH3 or SCH2CH(CH3)2; R3 is CH2CH2CH(NH2)COOH or H; R4 is H or Cl], e.g. methylthioadenosine (vitamin L2) or 5'-deoxy-5'- chloroadenosine as an active ingredient. The aforementioned compound as a composition for preventing and inhibiting various ulcers is added to a wide range of normal foods and drinks and suitably used in the form of the foods and drinks for inhibiting and preventing the ulcers. The dose thereof is normally about 0.001-40mg/kg body weight/day.