 All subjects who experience at least a 30 reduction in their weekly mean pain score during the single blind flexible dosing phase will be considered a responder, and will be further continued in the study. Those failing to achieve this level of pain relief will be discontinued from further study. Responders will be randomized to 5 weeks of double-blind treatment with nabilone or placebo. The double-blind phase of this study will permit confirmation of efficacy observed during the single-blind phase of the study. During the double-blind portion of the study, subjects randomized to nabilone will continue on the dose of nabilone achieved at the completion of the single-blind phase, and this dose will be maintained throughout the double-blind phase. Subjects randomized to placebo will receive a direct switch to placebo. All subjects will maintain their pre-study doses of analgesic medication for pain control throughout the study. 