Method for treating drug-induced constipation

1. A method for treating drug-induced constipation comprising a step of administering an effective amount of 15-keto-prostaglandin compound to a subject suffering from drug-induced constipation or in need thereof of such treatment, wherein the 15-keto-prostaglandin compound is one represented by formula (I):

• whereinL,M and N are hydrogen, hydroxy, halogen, lower alkyl, hydroxy(lower)alkyl or oxo, wherein at least one of L and M is a group other than hydrogen, and the five-membered ring may have at least one double bond;
  
  A is —
  
  CH₂OH, —
  
  COCH₂OH, —
  
  COOH or a functional derivative thereof;
  
  B is —
  
  CH₂—
  
  CH₂—
  
  , —
  
  CH═
  
  CH—
  
  or —
  
  C≡
  
  C—
  
  ;
  
  R₁ is a saturated or unsaturated bivalent lower or medium aliphatic hydrocarbon residue, which is unsubstituted or substituted with halogen, alkyl, hydroxy, oxo, aryl or heterocyclic group and at least one carbon atom in the aliphatic hydrocarbon is optionally substituted by oxygen, nitrogen or sulfur; and
  
  Ra is a saturated or unsaturated lower or medium aliphatic hydrocarbon residue, which is unsubstituted or substituted with halogen, oxo, hydroxy, lower alkoxy, lower alkanoyloxy, cyclo(lower)alkyl, cyclo(lower)alkyloxy, aryl, aryloxy, heterocyclic group or heterocyclic-oxy group;
  
  cyclo(lower)alkyl;
  
  cyclo(lower)alkyloxy;
  
  aryl;
  
  aryloxy;
  
  heterocyclic group;
  
  heterocyclic-oxy group.