A drug binds to the same receptor as an endogenous full agonist but produces a maximum response of only 40% of the maximal system response, even at saturating concentrations. When co-administered with a full agonist, this drug will:

• A Produce an additive increase in maximal response above that of the full agonist alone
• B Have no effect because the full agonist has higher affinity
• C Increase the potency of the full agonist without changing its maximal effect
• D Reduce the maximal response achievable by the full agonist ✓

Explanation

A partial agonist has lower intrinsic efficacy than a full agonist. Occupying receptors, it prevents the full agonist from binding those receptors, so the observed maximum falls toward the partial agonist's ceiling. This is why partial agonists such as buprenorphine at mu receptors can act as antagonists in patients dependent on full agonists like morphine, precipitating withdrawal. Affinity is irrelevant here; the interaction is competition at the receptor with differing efficacy.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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