The release of histamine is probably the reason for the vasodilatation seen with morphine. Histamine may also be responsible for the local urticaria seen with local injection. Pruritus associated with opioid use could be due to a central mechanism and is more likely to do with neuraxial administration (Giorgio et al. 2004). Naloxone and antihistaminics are useful in relieving pruritus, but not the histamine effects (Conti et al. 2005).
In summary, opioid pharmacology is still in its infancy and each day brings about improved appreciation of its diverse pathways and functions. We should expect an ever-increasing understanding of the structure-activity relationship, agonist-receptor dynamics, and clinically relevant therapeutics in the very near future.
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