What is the typical effect of a partial agonist when co-administered with a full agonist?

• A. It can reduce the overall maximal effect due to competition.
• B. It increases the maximal effect beyond the full agonist.
• C. It has no effect on the full agonist.
• D. It converts the full agonist into an antagonist.

Answer: (A)

Partial agonists have lower efficacy than full agonists. When both are present at the same receptors, the partial agonist competes for binding. Each receptor occupied by the partial agonist produces less response than one bound by the full agonist, so the overall activation is diminished. If the partial agonist has enough affinity, it can occupy a sizable portion of receptors and cap the maximum response the full agonist could achieve, lowering the apparent Emax. It doesn’t increase the response beyond what the full agonist can do, and it doesn’t convert the full agonist into an antagonist; instead, it acts as a competitive modulator that reduces the achievable maximal effect.