📖 Dose–Response Relationship (DRR)

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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 


The dose–response relationship describes how the intensity or probability of a drug’s effect changes with increasing dose. It is a cornerstone of pharmacology and helps define:

  • The minimum effective dose

  • The therapeutic (optimal) dose

  • The toxic or lethal dose

⚖️ Principle

As drug dose increases, the effect increases — up to a maximum. Beyond this, higher doses give no further therapeutic benefit but may increase toxicity.

🔹 Types of Dose–Response Relationships

1. Graded Dose–Response Relationship

  • Seen in a single individual.

  • Shows a continuous, incremental response as dose increases.

  • Useful for studying potency and efficacy.

Plots:

  • Dose vs Response → Rectangular hyperbola.

  • Log Dose vs Response → Sigmoid (S-shaped curve) → easier comparison of drugs.

Key Parameters:

  • Emax → maximum effect a drug can produce.

  • EC50 → concentration that produces 50% of max effect → measure of potency.

  • Slope → steepness of curve, indicates sensitivity (steep slope = narrow safety margin).

2. Quantal Dose–Response Relationship

  • Seen in a population.

  • Measures all-or-none effects (e.g., sleep vs no sleep, seizure prevention, death).

  • Data plotted as cumulative % of responders vs log-dose → normal distribution.

Key Measures:

  • ED50 → dose effective in 50% of population.

  • TD50 → dose toxic in 50%.

  • LD50 → dose lethal in 50% (animal studies).

  • Therapeutic Index (TI) = LD50 / ED50 → higher TI = safer drug.

🩺 Clinical Applications

  • Potency → lower dose needed for effect (e.g., fentanyl > morphine).

  • Efficacy → maximum benefit achievable; clinically more important than potency.

  • Therapeutic Index (TI) → guides drug safety; drugs with narrow TI (digoxin, warfarin, lithium) need close monitoring.

  • Tolerance → repeated doses shift DR curve to the right (higher dose needed).

  • Ceiling effect → beyond a certain dose, no added benefit, only more side effects.

📊 Examples

  • Salbutamol:

    • Graded → degree of bronchodilation in one patient.

    • Quantal → % of patients achieving ≥12% FEV1 improvement.

  • Paracetamol:

    • Graded → fall in temperature in °C.

    • Quantal → % of patients who become afebrile.

  • Diazepam:

    • Graded → depth of sedation.

    • Quantal → % of people who fall asleep.

🧭 Summary

  • Graded DRR = continuous effect in individuals (Emax, EC50, slope).

  • Quantal DRR = all-or-none effect in populations (ED50, TD50, LD50, TI).

  • Clinically useful for determining potency, efficacy, safety, and dosing strategies.

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