Pharmacokinetics of Drugs
How absorption, distribution, metabolism and elimination change in special patient populations
Pharmacokinetics describes what the body does to a drug — how it is absorbed, distributed through tissues, metabolised, and eliminated. The evaluation of these processes across different patient populations is essential, as age, organ function, body composition, and physiological state all alter how drugs behave. Understanding these changes prevents toxicity and ensures therapeutic efficacy.
What is pharmacokinetics?
Pharmacokinetics (PK) refers to the four key processes that determine a drug's fate in the body — absorption (how the drug enters the bloodstream), distribution (how it spreads through the body's tissues), metabolism (how it is chemically altered, mainly by the liver), and elimination (how it is removed from the body, mainly via the kidneys). Together these processes determine how much drug reaches its target, for how long, and at what concentration.
Why does it matter in special populations?
Standard adult drug doses are based on average pharmacokinetic data from clinical trials conducted predominantly in healthy adults. Special patient populations — including neonates and children, elderly patients, pregnant women, obese individuals, and those with renal or hepatic impairment — have significant physiological differences that alter one or more of these four PK processes. These differences can lead to under-dosing (therapeutic failure) or over-dosing (toxicity) if standard doses are used without adjustment.
Clinical application
Pharmacists play a central role in identifying patients who require dose adjustments based on their clinical characteristics. Tools such as the Cockcroft-Gault equation (for renal dosing), the Child-Pugh score (for hepatic impairment), and weight-based dosing scalars (for paediatrics and obesity) help quantify the degree of adjustment needed. Use the evaluation trigger to access a detailed summary of how each PK parameter changes across the six main special population groups, with clinical examples and dosing implications.