A 54-year-old man who was in the hospital after a surgical procedure complained of a severe abdominal pain.  The nurse practitioner decided to start analgesic treatment with an opioid.  The drugs he was considering were morphine (10 mg IM) and buprenorphine (0.3 mg IM). Morphine is a full agonist at mu (μ) opioid receptors, whereas buprenorphine is a partial agonist at the same receptors.  The above-mentioned doses of the two drugs are equieffective.   Which of the following pairs of statements correctly defines the potency and efficacy of morphine and buprenorphine?

A. Morphine is more potent. Buprenorphine is more effective.
B. Morphine is more potent. Buprenorphine is less effective.
C. Morphine is less potent. Buprenorphine is more effective.
D. Morphine is less potent. Buprenorphine is less e ective.

Please provide rationales for your selected answer choice.

B. Morphine is more potent. Buprenorphine is less effective.

Step-by-step explanation

• The potency of an opioid is its ability to produce a response at a given receptor, whereas the efficacy of an opioid is its ability to produce a maximal response at a given receptor. Morphine is more potent than buprenorphine, but buprenorphine is more effective because it produces a greater response at a given receptor than morphine.
• For a given concentration, morphine will produce a greater maximal response than buprenorphine; therefore, morphine is more potent. In addition, for a given receptor occupancy, the intensity of the response produced by buprenorphine at μ opioid receptors will be greater than the response produced by equimolar concentrations of morphine because of its competitive advantage at the receptor; therefore, it is more effective.
• Potency is associated with the concentration of a drug that is needed to trigger a response, while efficacy is associated with the strength of the response that a drug can trigger.  Morphine is more potent, while buprenorphine is less potent.  The potency of a drug is related to its ability to bind to and trigger receptors.  Higher potency means lower drug concentrations are needed and thus higher receptor occupancy.
• Morphine is a potent, short-acting opiod that produces predictable effects.  It is considered to be the gold standard for epidural administration during labor and for postoperative analgesia.  You should consider using morphine for patients who are responsive to this agent and who do not require high doses of long-acting opioids such as methadone and buprenorphine (Buprenex).