⚡ Drug Potency & Efficacy: Core Concepts in Pharmacodynamics

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By Dr. Sk Sabir Rahaman, MBBS, MD (Pharmacology), DFM(Family Medicine), FCFM, CCEBDM, CCLSD 

📍 Specialist Family Physician | Consultant Pharmacologist | Lifestyle & Diabetes Expert 


Drugs differ not only in how much is needed to produce an effect but also in how much effect they can ultimately produce. These differences are explained by potency and efficacy.

🔹 1. Drug Potency

📌 Definition:
Potency is the dose (or concentration) of a drug required to produce a given effect, usually expressed as EC₅₀ (the dose that gives 50% of maximum effect).

  • A more potent drug achieves the same effect at a lower dose.

  • Potency is dose efficiency, not drug strength.

📊 Example:

  • Morphine (10 mg) and Pethidine (100 mg) give equivalent analgesia.
    ➡ Morphine is 10× more potent than Pethidine.

📈 Graphical Insight:

  • On a log dose–response curve (DRC):

    • Leftward shift → higher potency

    • Rightward shift → lower potency

👉 Clinical relevance: Potency matters in dose selection and prescribing.

🔹 2. Drug Efficacy

📌 Definition:
Efficacy is the maximum therapeutic effect (Eₘₐₓ) a drug can achieve, regardless of dose.

  • Reflects the intrinsic activity of the drug.

  • Indicates the ceiling effect beyond which higher doses won’t help.

📊 Example:

  • Morphine relieves severe pain.

  • Aspirin relieves only mild-to-moderate pain.
    ➡ Morphine is more efficacious than aspirin.

📈 Graphical Insight:

  • The height of the DRC reflects efficacy.

  • A drug can be highly potent but less efficacious (or vice versa).

👉 Clinical relevance: Efficacy is more important than potency when selecting drugs for therapy.

📊 Potency vs Efficacy: Key Differences

FeaturePotencyEfficacy
DefinitionDose needed for a given effectMaximum effect achievable
MetricEC₅₀ (or ED₅₀)Eₘₐₓ
Graphical RepresentationLeft–right shift of DRCHeight of DRC
Clinical ImportanceGuides dose selectionGuides drug choice
ExampleMorphine more potent than PethidineMorphine more efficacious than Aspirin

✨ Key Takeaways

  • Potency = “How much drug?”

  • Efficacy = “How good is the drug at doing its job?”

  • For clinical decision-making:

    • Efficacy determines which drug to use.

    • Potency determines what dose to prescribe.

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