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Pharmacodynamics

Codeine is a nonsynthetic opioid. It is a selective agonist of the μ-opioid receptor (MOR). Codeine itself has relatively weak affinity for the MOR. Instead of acting directly on the MOR, codeine functions as a prodrug of its major active metabolites morphine and codeine-6-glucuronide, which are far more potent MOR agonists in comparison.