6. Treatment with opioids (e.g., tramadol, meperidine derivatives), serotonin norepinephrine reuptake inhibitors (SNRIs) (e.g. venlafaxine, duloxetine), tri- or tetra-cyclic antidepressants, in the past 8 weeks prior to the screening visit. Dextromethorphan will be permitted if used for treating cough.