Analgesic agent

A method of enhancing the analgesic activity of morphine when administered to an animal feeling pain, which comprises administering an effective and non-toxic amount of the compound of the formula (I) ##STR15## wherein R.sub.1 denotes a hydrogen atom or a hydroxyl group, X denotes a group --COR.sub.2 where R.sub.2 is a hydroxyl group, a lower alkoxyl group, a benzyloxy group, an amino group or a lower alkyl mono- or di-substituted amino group, or X denotes a group --CH.sub.2 OH, a group ##STR16## where R.sub.3 and R.sub.4 are equal to each other or are different from each other and are each a hydrogen atom, an alkyl group of 1 to 7 carbon atoms, a hydroxyalkyl group containing 1 to 7 carbon atoms, a mercaptoalkyl group containing 1 to 7 carbon atoms, a carboxamidoalkyl group containing 2 to 8 carbon atoms, an aminoalkyl group containing 1 to 7 carbon atoms, a guanidyl-N-alkyl group containing 2 to 4 carbon atoms, an alkylmercaptoalkyl group containing 2 to 8 carbon atoms, a carboxyalkyl group containing 2 to 8 carbon atoms, an aryl group, especially phenyl, an aralkyl group, especially phenyl-(C.sub.1 -C.sub.4) alkyl, or a substituted aralkyl group, and Y is a group --CH.sub.2 OH, a group --COR.sub.2 or a group --CH.sub.2 COR.sub.2 where R.sub.2 is as defined above,

• n is zero or 1, and the asymmetric carbon atoms present in the compound may take the R-configuration or the S-configuration or a combination thereof to the animal just before or at the same time as when morphine is given to the animal for the analgesic purpose.