Low doses of risperidone and morphine interact to produce more analgesia and greater extrapyramidal effects in rats

▶ Risperidone enhances the effects of morphine on pain sensitivity in rats. ▶ Risperidone also induces atypical motor behavior. ▶ Motor effects occur at lower doses than does enhancement of morphine analgesia. ▶ Morphine enhances risperidone-induced atypical motor effects. ▶ The utility of risperidone as an adjuvant to opioid treatment of pain is limited.