Pharmacology Terms: Affinity, Efficacy & Potency

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  • 0:02 Confusing Terms
  • 0:41 Affinity
  • 1:37 Efficacy
  • 2:31 Potency
  • 3:35 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

How do we determine how well a drug works? This lesson will discuss three important and often confused properties of drugs, affinity, efficacy, and potency, and how they differ from one another.

Confusing Terms

Have you ever had an affinity for someone only to find your efforts weren't effective in getting them to be your friend?

Affinity and efficacy are not the same thing. Affinity is a relatively passive process. You may like someone, but so what? How effective is it to just like someone in order to get them to be your friend? Not very.

And if you did succeed in becoming friends because you were effective, then your relationship may or may not have been potent, or powerful.

Affinity, efficacy, and potency are terms that are also applicable, believe it or not, to drugs. Let's see how.


Drugs work by binding to specific receptors and activating them, causing a downstream effect.

Affinity is how avidly a drug binds its receptor or how the chemical forces that cause a substance to bind its receptor.

Affinity is like a drug's desire to connect to an open receptor. It refers to how much attraction there is between a drug and a receptor, like a magnet to metal.

Some drugs have higher affinity and others have a lower affinity for the same binding sites.

Let's say that a drug with a low affinity for a receptor has bound to said receptor. Then, all of the sudden, another drug comes along with a much higher affinity for the receptor. The second drug will displace the lower affinity drug and bind to the receptor itself because it wants it that much more!

This may remind you of those holiday shopping scenes where parents with the highest affinity for the newest children's toy push others out of the way to grab the last toy in the store.


Now, affinity is not the same as efficacy.

Efficacy (intrinsic activity)in this case refers to the drug's ability to activate the receptor once it has bound to it.

When a drug binds a receptor, assuming it does so effectively, it will turn the receptor 'on,' or activate it. Activation of the receptor will initiate a domino effect of reactions that eventually causes the intended result of the drug, such as the slowing down of heart rate.

Some drugs have high affinity and high efficacy. This means they bind the receptor with a great desire and activate the receptor to do its job really well. That's like saying a carpenter holds on to, or has a high affinity for, a drill and is effective at using it to build a home.

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