Nalbuphine Is an Effective Pain Reliever Because of Its Activity

Nalbuphine is an effective pain reliever because of its activity at mu-opioid receptors. However, if given to a patient who has recently received morphine (which also affects mu-opioid receptors) for postoperative pain, nalbuphine can worsen his or her pain. Thus nalbuphine is said to have what kind of activity at mu-opioid receptors?

A) Full agonist
B) Partial agonist
C) Competitive antagonist
D) Non-competitive antagonist
E) Allosteric modifier