Patent ID: 8865731

Claim:
A method for suppressing an undesirable effect of an opioid, which comprises administering an effective amount of a compound represented by at least one of formulae (I) to (VIII), or a pharmaceutically acceptable salt thereof: in which R 1 represents a hydrogen atom or methyl; R 2 and R 3 independently represent methyl, ethyl, propyl, butyl or isopropyl; R 4 , R 5 and R 6 independently represent a hydrogen atom, methyl, ethyl, methoxy, ethoxy, a fluorine atom, a chlorine atom or a bromine atom; R 7 represents methyl, ethyl, propyl, butyl or 3-methylbutyl, each of which is optionally substituted with hydroxy; R 8 represents phenyl, pyridyl, pyrimidinyl or 5,6-dihydro-2H-pyridylmethyl, each of which is optionally substituted with 1 to 3 substituents selected from the group consisting of a chlorine atom, methyl, ethyl, methoxy and ethoxy; R 9 represents pyridyl or tetrahydropyranyl; R 10 and R 11 independently represent a hydrogen atom, a fluorine atom or 2-methoxyethoxy; and R 12 represents methyl, ethyl, propyl or butyl, wherein said undesirable effect is analgesic tolerance.