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Medical Science
Grade 11-12
Pharmacokinetics ADME Reference Cheat Sheet
A printable reference covering ADME, bioavailability, volume of distribution, clearance, half-life, loading dose, and maintenance dose for grades 11-12.
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Pharmacokinetics explains how a drug moves through the body through absorption, distribution, metabolism, and excretion, often shortened to ADME. This cheat sheet helps students connect medical science vocabulary with the formulas used to estimate drug levels over time. It is useful for understanding why dose, route, liver function, kidney function, and timing can change a drug’s effect. The goal is to make core ADME ideas clear enough to support safe reasoning about medicines in clinical examples.
Key Facts
- Bioavailability is F = amount reaching systemic circulation / amount administered, and IV drugs usually have F = 1 or 100%.
- Volume of distribution is Vd = amount of drug in body / plasma drug concentration.
- Clearance is CL = rate of drug elimination / plasma drug concentration.
- Half-life for first-order elimination is t1/2 = 0.693 x Vd / CL.
- Loading dose is LD = target plasma concentration x Vd / F.
- Maintenance dose rate is dosing rate = target plasma concentration x CL / F.
- At steady state, the average drug input rate equals the average drug elimination rate.
- For most first-order drugs, about 4 to 5 half-lives are needed to reach near steady state or near complete elimination.
Vocabulary
- Absorption
- Absorption is the movement of a drug from its site of administration into the bloodstream.
- Bioavailability
- Bioavailability is the fraction of an administered drug dose that reaches systemic circulation unchanged.
- Volume of distribution
- Volume of distribution is an apparent volume that relates the amount of drug in the body to the measured plasma concentration.
- Clearance
- Clearance is the volume of plasma cleared of drug per unit time by metabolism, excretion, or both.
- Half-life
- Half-life is the time needed for the plasma drug concentration to decrease by 50%.
- Steady state
- Steady state is the condition in which average drug input equals average drug elimination, making average plasma concentration stable.
Common Mistakes to Avoid
- Confusing absorption with bioavailability is wrong because a drug can be absorbed from the gut but still be reduced by first-pass metabolism before reaching systemic circulation.
- Treating volume of distribution as a real body volume is wrong because Vd is an apparent value based on plasma concentration and tissue binding.
- Assuming a higher dose always means a longer half-life is wrong because half-life depends mainly on Vd and clearance for first-order drugs.
- Forgetting to include F in oral dosing calculations is wrong because incomplete bioavailability means only part of the dose reaches systemic circulation.
- Thinking steady state occurs after one dose is wrong because most first-order drugs need about 4 to 5 half-lives to approach steady state.
Practice Questions
- 1 A 500 mg oral dose has bioavailability F = 0.40. How many milligrams reach systemic circulation unchanged?
- 2 A drug has Vd = 40 L and CL = 5 L/hr. Calculate its half-life using t1/2 = 0.693 x Vd / CL.
- 3 A target plasma concentration is 10 mg/L, Vd is 30 L, and F is 0.75. Calculate the loading dose using LD = target concentration x Vd / F.
- 4 A patient with severe kidney disease is taking a drug mainly eliminated by the kidneys. Explain how reduced clearance could affect half-life and dosing safety.