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.
Understanding Pharmacokinetics ADME Reference
Drug concentration is not the same as drug effect. A medicine must reach its target tissue, bind to a receptor, and cause a biological change. Some drugs act soon after blood levels rise.
Others have a delay because they change protein production, inflammation, or cell growth. The useful concentration range lies between a level that is too low to work and a level high enough to cause harmful effects.
This range is called the therapeutic window. Drugs with a narrow therapeutic window need especially careful dosing because a small concentration change can matter.
The route of administration can strongly change the time course. An intravenous dose enters the bloodstream immediately. A tablet must dissolve, pass through the gut wall, then travel through the liver before reaching the general circulation.
The liver may break down part of the dose during this first pass. Food can slow stomach emptying or change absorption. Vomiting, diarrhea, gut disease, and poor blood flow can make oral absorption less predictable.
A skin patch releases drug slowly, while an inhaled drug can reach the blood rapidly through the lungs. These differences explain why equal milligram doses by different routes may not have equal effects.
Volume of distribution is a calculated value, not the actual physical volume of a person. It shows how much a drug appears to leave the blood and enter tissues. A drug that stays mainly in plasma tends to have a smaller volume of distribution.
A fat soluble drug may collect in fat or other tissues and have a larger value. Blood protein binding matters too.
Only unbound drug can usually cross membranes, be filtered by the kidneys, or interact with targets. Low blood protein levels can raise the unbound fraction of some medicines, increasing effects even when the measured total concentration looks unchanged.
Clearance describes the body’s ability to remove drug from the blood over time. The kidneys remove many drugs by filtration, active secretion, or both. The liver can chemically alter drugs into metabolites that may be inactive, active, or harmful.
Reduced kidney function often lowers clearance of drugs removed in urine. Liver disease can slow metabolism, especially for drugs with extensive liver processing. Drug interactions are important because one medicine can block or speed up enzymes that process another.
In first-order elimination, a constant fraction is removed during each half-life. Students should distinguish a fraction from a fixed amount. A changing half-life, severe organ impairment, or a very narrow therapeutic window means that simple estimates need extra caution.
Loading doses are used when a useful concentration is needed quickly, such as when starting certain antibiotics or heart rhythm medicines. They depend mainly on the target concentration and volume of distribution. Maintenance dosing replaces what is removed after treatment has begun.
It depends mainly on clearance. Repeating a dose before the previous dose has fully left the body causes accumulation until average input matches average removal. Missing doses, taking doses too close together, or changing kidney function can shift this balance.
In clinical practice, measured drug levels, symptoms, side effects, age, body size, and organ function guide decisions. Formula results are estimates that support judgment, not automatic instructions for self-dosing.