 Morphine is a phenanthrene opioid receptor agonist – its main effect is binding to and activating the μ-opioid receptor (MOR) in the central nervous system.Its intrinsic activity at the MOR is heavily dependent on the assay and tissue being tested; in some situations it is a full agonist while in others it can be a partial agonist or even antagonist.In clinical settings, morphine exerts its principal pharmacological effect on the central nervous system and gastrointestinal tract.