biological variability
Give ten people the very same dose of the very same drug and you will get ten somewhat different responses. Biological variability is this spread of reactions among individuals receiving identical doses — the reason medicine is never one-size-fits-all.
The variation has two broad sources. Pharmacokinetic variability changes how much drug reaches the site of action — differences in absorption, distribution, metabolism (often genetic, as with CYP enzyme polymorphisms), and excretion alter the concentration achieved from the same dose. Pharmacodynamic variability changes the response to a given concentration — differences in receptor number, sensitivity, or downstream physiology. Age, organ function, body composition, disease, other drugs, and genetics all contribute.
Biological variability is precisely why quantal dose–response curves have a spread rather than a single step, why a population has an ED50 instead of one universal effective dose, and why doses must often be individualized by titration or guided by therapeutic drug monitoring. Recognizing it underpins special-population dosing and the whole field of personalized medicine.
Warfarin doses can differ more than tenfold between patients to reach the same INR, largely because of CYP2C9 and VKORC1 genetic variation — a textbook case of biological variability.
Same target, very different doses.
Variability is not noise to be ignored — it is information. Wide variability in response is itself a reason to favour titration, monitoring, or pharmacogenetic testing over a fixed standard dose.