Side effect of high dose epidural opioid: Nausea and vomiting Pruritis Urinary retention Respiratory depression High dose of opioid can spread centrally and stimulate CTZ center and can precipitate nausea and vomiting. Pruritis due to morphine is not due to histamine release, high dose morphine can spread centrally, act on trigeminal nerve and cause pruritis. Urinary retention in morphine use is due to it's action on sacral spinal cord, therefore it can happen even at normal dose and not related to high dose. Respiratory depression in morphine overdose due to migration of morphine centrally.