1. What is Plasma Half-Life?
**Plasma Half-Life (t½)** is the time required for the plasma concentration of a drug to reduce to **one-half (50%)** of its original value. It indicates the rate of drug elimination.
A 2-hour half-life means concentration drops by 50% every 2 hours (in First Order Kinetics).
2. The Core Formulas
Primary formula relating to **Vd** and **Clearance**:
Derived from **Elimination Rate Constant ($k_{el}$)**:
$k_{el}$ is the **fraction** eliminated per unit time. Higher $k_{el}$ = Shorter $t_{1/2}$.
3. Kinetics of Elimination
Most dental drugs follow First Order Kinetics.
A. First Order (Linear)
- Constant **FRACTION (%)** eliminated.
- Half-Life is **CONSTANT**.
- Examples: Amoxicillin, Lignocaine.
B. Zero Order (Saturation)
- Constant **AMOUNT (mg)** eliminated.
- Half-Life **INCREASES** with dose.
- Examples: Alcohol, Phenytoin (high dose).
4. The "Rule of 5"
It takes **4 to 5 half-lives** for clinical elimination (>96% removed) or to reach steady state.
Drug Eliminated
5. Factors Affecting t½
INCREASES t½
Stays Longer
- Renal Failure
- Liver Disease
- High Protein Binding
- Enzyme Inhibition
DECREASES t½
Leaves Faster
- Enzyme Induction (Smoking)
- Protein Displacement
- Alkaline/Acidic Urine
6. Clinical Significance
Dosing Freq
Short t½ = Frequent dosing.
Long t½ = Once daily.
Steady State
Reached after 4-5 half-lives.
Loading Dose
Used for long t½ drugs to act fast.
Toxicity
Predicts monitoring duration.
7. Dental Relevance
Local Anesthetics (min)
Lignocaine (~90m) vs Articaine (~20m). Articaine clears faster = safer for re-injection.
Antibiotics (hours)
Amoxicillin (~1h, TDS) vs Azithromycin (~68h, OD). Long t½ aids compliance.
8. Calculations
Q: Drug t½ = 4 hrs. Time to 94% elimination?
A: 4 half-lives × 4 hrs = 16 hours.
Q: Vd = 100L, CL = 10L/hr. Calculate t½.
A: (0.693 × 100) / 10 = 6.93 hours.
