This invention is directed to a method for preventing or treating neuropathic pain and cluster and migraine headache-associated pain comprising administering to a subject in need thereof a therapeutically effective amount of an enantiomer of Formula (I) substantially free of other enantiomers or an enantiomeric mixture wherein an enantiomer of Formula (I) predominates: ##STR00001## wherein phenyl is substituted at X with one to five halogen atoms independently selected from the group consisting of fluorine, chlorine, bromine and iodine; and; R.sub.1 and R.sub.2 are independently selected from the group consisting of hydrogen and C.sub.1-C.sub.4 alkyl; wherein C.sub.1-C.sub.4 alkyl is optionally substituted with phenyl (wherein phenyl is optionally substituted with substituents independently selected from the group consisting of hydrogen, halogen, C.sub.1-C.sub.4 alkyl, C.sub.1-C.sub.4 alkoxy, amino, nitro and cyano).