True anaphylactic reactions to opioids are very rare. When patients say they are allergic to an opiod, it is much more likely that the patient has experienced GI upset or a pseudoallergy.
Flushing, itching, hives, and sweating, especially itching or flushing at the injection site only, suggests a pseudoallergy due to histamine release, a pharmacologic side effect of some opioids. Codeine, morphine, and meperidine are the opioids most commonly associated with pseudoallergy. Use of a more potent opioid is less likely to result in histamine release.
According to Emergency Physicians Monthly, "It would also seem that if the adverse reactions to opioids are due to histamine release that administering the opioids with an antihistamine such as Phenergan, Vistaril, or Benadryl would serve both to stop the “allergic” reaction and to enhance the effect of the pain medication."