AIM • STEP 10
Student Memory Support
Drug Excretion, Dosage Regimens and Pharmacokinetic Principles
3rd Year MBBS • Pharmacology • High-Yield Memory Reinforcement & Rapid Revision
1. High-Yield Flashcards
Tap each question to reveal the answer.
What is drug excretion?
Physical removal of a drug or its metabolites from the body.
How does elimination differ from excretion?
Elimination includes metabolism plus excretion; excretion refers to physical removal.
What does drug clearance represent?
The volume of plasma cleared of drug per unit time.
What is the basic formula for drug clearance?
Clearance = rate of elimination ÷ plasma drug concentration.
What is the major route of excretion for many drugs and their metabolites?
Renal excretion through the kidneys.
Which renal processes determine urinary drug excretion?
Glomerular filtration, tubular secretion and tubular reabsorption.
What is the purpose of a maintenance dose?
To replace eliminated drug and maintain the desired plasma concentration.
Which factors determine maintenance dose?
Target concentration, clearance, dosing interval and bioavailability.
Which pediatric dose formula uses body weight in pounds?
Clark’s formula.
Which pediatric dose formulae are based on age?
Young’s formula and Dilling’s formula.
What is plasma half-life?
The time required for plasma drug concentration to decrease by 50%.
How are volume of distribution and clearance related to half-life?
t½ = 0.693 × Vd ÷ clearance.
What happens to half-life when clearance decreases while Vd remains unchanged?
The half-life increases.
When is steady state achieved during repeated dosing?
When the rate of drug input equals the rate of drug elimination.
Approximately how long does a first-order drug take to approach steady state?
About 4–5 half-lives.
What is eliminated at a constant rate in first-order kinetics?
A constant fraction of the drug per unit time.
What is eliminated at a constant rate in zero-order kinetics?
A constant amount of drug per unit time.
Why can a small increase in phenytoin dose cause a large rise in plasma concentration?
Its elimination can become capacity limited at higher concentrations.
2. Mnemonics
Mnemonic Title: Renal Drug Handling
FSR
Meaning: Filtration → Secretion → Reabsorption.
Mnemonic Title: Pediatric Dose Formulae
YDC
Meaning: Young = age; Dilling = age; Clark = weight.
Mnemonic Title: First vs Zero Order
F = Fraction, Z = Fixed
Meaning: First-order removes a constant fraction; zero-order removes a fixed amount per unit time.
3. Memory Tables
First-Order vs Zero-Order Kinetics
| Feature | First-Order | Zero-Order |
|---|---|---|
| Removed per unit time | Constant fraction | Constant amount |
| Elimination rate | Concentration-dependent | Capacity-limited |
| Half-life | Relatively constant | Not constant in the same way |
| Example | Most drugs | Phenytoin at higher concentrations |
Young’s, Dilling’s and Clark’s Formulae
| Formula | Basis | Calculation |
|---|---|---|
| Young’s | Age | Age ÷ (Age + 12) × adult dose |
| Dilling’s | Age | Age ÷ 20 × adult dose |
| Clark’s | Weight in lb | Weight in lb ÷ 150 × adult dose |
4. Rapid Revision Points — Last-Minute Revision
Must Remember:
- Excretion is physical removal; elimination includes metabolism plus excretion.
- Clearance = elimination rate ÷ plasma concentration.
- Total body clearance reflects the contribution of eliminating organs.
- Reduced renal clearance can increase drug accumulation when dosing is unchanged.
- Maintenance dose replaces drug lost through elimination.
- Lower bioavailability requires a larger administered maintenance dose for the same systemic input.
- t½ = 0.693 × Vd ÷ clearance.
- A longer half-life means a longer time to approach steady state.
- Steady state occurs when drug input equals drug elimination.
- First-order = constant fraction; zero-order = constant amount eliminated per unit time.
5. Clinical Memory Hooks
Renal function falls during drug therapy → reduced clearance → increased accumulation if the maintenance regimen is unchanged.
Long-half-life drug → slower approach to steady state → slower response after a maintenance-dose change.
Small increase in phenytoin dose followed by a large concentration rise → capacity-limited elimination at higher concentrations.
Repeated fixed dosing → progressive accumulation → input eventually equals elimination at steady state.
6. Starred High-Yield Exam Points
- ⭐ Clearance is the volume of plasma cleared of drug per unit time.
- ⭐ Maintenance dose is directly related to target concentration and clearance and inversely related to bioavailability.
- ⭐ Half-life increases with Vd and decreases with clearance: t½ = 0.693 × Vd/Cl.
- ⭐ First-order drugs approach steady state in approximately 4–5 half-lives.
- ⭐ First-order kinetics removes a constant fraction; zero-order kinetics removes a constant amount per unit time.
- ⭐ Saturation of phenytoin elimination can cause a disproportionate rise in plasma concentration after a small dose increase.