Piperidine derivatives which are compounds of formula (I) ##STR1## wherein Ar.sub.1 and Ar.sub.2 independently represent a phenyl group optionally substituted by a halogen atom, a thienyl group or a pyridinyl group, A represents an alkylene group of 2 to 6 carbon atoms, X represents a CO group or a bond, R.sub.1 represents a hydrogen atom or an alkyl group of 1 to 4 carbon atoms, either R.sub.2 represents a hydrogen atom and R.sub.3 represents a hydrogen atom or a hydroxy group, or R.sub.2 and R.sub.3 together represent a bond, and R.sub.4 represents a hydrogen or halogen atom, or pharmaceutically acceptable acid addition salts thereof are useful as antiallergics and antipruritics.