Explain the difference between affinity and efficacy in receptor pharmacology.

• A. Affinity is the binding strength between a drug and receptor; efficacy is the ability of the bound drug to activate the receptor and produce a response.
• B. Affinity means maximum effect; efficacy means binding strength.
• C. Affinity and efficacy are the same.
• D. Affinity is receptor activation; efficacy is receptor binding.

Answer: (A)

Affinity is about how tightly a drug binds to its receptor. It’s a binding property and doesn’t say anything about what happens after the drug is bound. Efficacy, on the other hand, is about what happens once the drug is bound: the ability to activate the receptor and produce a cellular response.

So you can have a drug that binds very tightly (high affinity) but, when it binds, only triggers a small response (low efficacy). That’s why such a drug can behave as a partial agonist or even an antagonist in some situations. Conversely, a drug might not bind very tightly (lower affinity) but, when it does bind, it can elicit a strong response (high efficacy). The key idea is that binding strength and the resulting activation are related but distinct properties. Affinity tells you about binding, while efficacy tells you about the functional outcome of that binding.