<p>The active agent comprises polyprenyl alcohol (or its ester) of formula <IMAGE> wherein n represents an integer of 7 to 10 and A a group of the general formula: <IMAGE> in which a and b each represent hydrogen or form a bond, or <IMAGE> where R represents hydrogen or -COR', R' being a saturated or unsaturated alkyl group having 1 to 17 carbon atoms; a cyclohexyl group; an unsubstituted phenyl group or a phenyl group substituted with a halogen atom or a lower alkoxy group; a (halogen atom- substituted phenoxy)-lower alkyl group; or a pyridyl group.</p>
