Pharmacology for Dentists — Chapter 2: Pharmacodynamics & Pharmacokinetics
“Drugs don’t act in isolation — they interact with who we are, where we are, and what we need.”
🧬 What Are PD and PK?
Pharmacodynamics (PD) answers: “What does the drug do to the body?”
Pharmacokinetics (PK) answers: “What does the body do to the drug?”
These two principles are the foundation of why a drug works, how fast it works, and how long it lasts.
Pharmacodynamics: Mechanism Meets Meaning
Pharmacodynamics describes how drugs exert their effects on the body through interactions with cellular targets. It is the science that connects molecular action to clinical outcomes. Whether a drug activates or blocks a receptor, increases or suppresses a signaling cascade, or mimics a physiological molecule, pharmacodynamics explains how those actions translate into healing—or harm. It’s the reason one drug lowers blood pressure, while another relieves pain or induces sleep.
PD explores:
- Receptor interactions (agonist, antagonist, partial agonist)
- Dose-response relationships
- Therapeutic window and efficacy
Key terms:
- Efficacy = Maximum effect a drug can achieve (height of curve)
- Potency = Amount of drug needed to produce an effect (shift on X-axis)
- Therapeutic Index (TI) = LD50 / ED50 → the higher, the safer
🧠 Example: Morphine has high efficacy but low potency compared to fentanyl.
PD explores:
- Receptor interactions (agonist, antagonist, partial agonist)
- Dose-response relationships
- Therapeutic window and efficacy
Key terms:
- Efficacy = Maximum effect a drug can achieve (height of curve)
- Potency = Amount of drug needed to produce an effect (shift on X-axis)
- Therapeutic Index (TI) = LD50 / ED50 → the higher, the safer
🧠 Example: Morphine has high efficacy but low potency compared to fentanyl.
🚚 Pharmacokinetics: The Journey of a Drug
Pharmacokinetics is the science of a drug’s journey through the body. It tells us how the body handles a substance—from the moment it’s taken in, through its distribution and transformation, to its eventual elimination. Just like a traveler passing through customs, transport hubs, and exit points, a drug faces barriers, checkpoints, and metabolic processing before it can reach its destination and exert its effect.
Understanding PK gives insight into how quickly a drug works, how often it needs to be taken, and how it behaves in different tissues or in patients with impaired metabolism or excretion.
PK involves four major steps — ADME:
- Absorption – How the drug enters the bloodstream
- Distribution – Where the drug goes in the body
- Metabolism – How the drug is broken down (mostly liver)
- Excretion – How the drug leaves the body (urine, bile, etc.)
Key concepts:
- Bioavailability (F) = Fraction of unchanged drug reaching systemic circulation
- First-pass metabolism = Liver inactivation before reaching circulation (e.g., oral nitroglycerin)
- Volume of distribution (Vd) = Theoretical space a drug occupies
- Half-life (t1/2) = Time to reduce plasma concentration by 50%
- Steady state = When drug input = drug elimination (achieved in ~4-5 half-lives)
PK involves four major steps — ADME:
- Absorption – How the drug enters the bloodstream
- Distribution – Where the drug goes in the body
- Metabolism – How the drug is broken down (mostly liver)
- Excretion – How the drug leaves the body (urine, bile, etc.)
Key concepts:
- Bioavailability (F) = Fraction of unchanged drug reaching systemic circulation
- First-pass metabolism = Liver inactivation before reaching circulation (e.g., oral nitroglycerin)
- Volume of distribution (Vd) = Theoretical space a drug occupies
- Half-life (t1/2) = Time to reduce plasma concentration by 50%
- Steady state = When drug input = drug elimination (achieved in ~4-5 half-lives)
📊 Why Dentists Should Care
- Choosing ibuprofen vs. acetaminophen? That’s PK + PD.
- Timing an antibiotic before surgery? PK.
- Choosing lidocaine over articaine? Both.
Understanding PD/PK helps dentists:
- Avoid overdose
- Choose correct dosing intervals
- Interpret delayed or rapid effects
- Personalize care for children, elderly, hepatic or renal patients
🧭 In Summary
| Term | Meaning |
|---|---|
| Efficacy | Max effect (Y-axis) |
| Potency | Dose to achieve effect (X-axis) |
| Bioavailability | Fraction absorbed |
| Half-life | Duration of effect |
| Vd | How widely drug spreads |
| TI | Safety margin |
👉 Next Chapter
Read: Chapter 3: Autonomic Drugs — Navigating Alpha, Beta, and Beyond
