4 Drug Action and Variability in Response
Learn how drugs act on biological targets, how dose–response measures describe their effects, and why medication responses vary among people.
Drug targets and receptor action
describes what a drug does to the body: how it interacts with a target and how that interaction changes a physiological response. —absorption, distribution, metabolism, and excretion—affects the concentration of drug reaching its site of action. A person's response reflects both drug exposure and the sensitivity of the body's targets and systems.
Many drugs act at receptors, proteins that bind endogenous signaling molecules or drugs. Other drugs act on enzymes, ion channels, transporters, structural components, or chemical processes without binding a receptor. For example, a receptor agonist can imitate a body signal, an enzyme inhibitor can slow a biochemical reaction, and an ion-channel blocker can alter the movement of charged particles across a cell membrane.
and describe different properties of drug action. is a drug's tendency to bind a target, while is its ability, once bound, to produce a response. Binding alone does not necessarily activate a target.
Types of receptor action
A full agonist binds to a receptor and can produce the system's full response.
A activates the receptor but produces a smaller maximum response than a full agonist in the same system. In the presence of a full agonist, it may reduce the overall response by competing for receptors while activating them less strongly.
A binds without activating the receptor and prevents an agonist from binding. When binding is reversible, increasing agonist concentration can often overcome the blockade.
A reduces the response in a way that increasing agonist concentration cannot fully overcome. It may act irreversibly or through a distinct binding site or mechanism.
An reduces a receptor's baseline activity when the receptor is active even without an agonist. Unlike a neutral antagonist, it changes this baseline activity.
These descriptions are useful models; observed effects can depend on the tissue, receptor system, and experimental or clinical context.
Dose–response and therapeutic safety
A dose–response relationship describes how an effect changes as a dose or drug concentration changes. For an individual, increasing the dose often increases the effect over part of the range, until the response approaches a maximum. Dose–response relationships also help describe the risk of unwanted or toxic effects.
is how much drug is needed to produce a specified effect. On a concentration–response graph, a more potent drug generally reaches that effect at a lower concentration. is the maximum effect a drug can produce in the system; greater does not necessarily mean greater .
For example, Drug A may produce a given degree of pain relief at a lower dose than Drug B, making A more potent for that measured effect. If Drug B can produce a greater maximum effect, B is more efficacious. and comparisons depend on the effect and system being measured.
Curves and measures
A graded dose–response curve measures the size of an effect as dose or concentration changes. The concentration that produces half of the measured maximum effect is denoted ; it is commonly used as a measure of in a particular assay. A instead records whether a defined effect occurs in each member of a population, such as the proportion of people who respond at each dose.
The therapeutic window is the range of exposures in which a drug is expected to provide benefit without unacceptable toxicity. The compares doses associated with toxicity and therapeutic effects in a population. It is a general safety measure, not a guarantee that a particular dose is safe for every person. Drugs with a narrow therapeutic window may require closer monitoring and careful dose adjustment.
Why medication responses vary
People can respond differently to the same medication and dose. Variation may change the amount of drug reaching its target, the body's response at a given concentration, or both.
Important influences include:
Age and body composition: Development, aging, and differences in body water, fat, and organ function can alter drug distribution, clearance, or sensitivity. Effects differ among drugs; age alone does not predict a uniform response.
Genetics: Variants in drug-metabolizing enzymes, transporters, or drug targets may change exposure or effect. Pharmacogenetic information is relevant to particular drug–gene pairs, not automatically to every medication or patient.
Organ function and disease: Liver or kidney impairment can change metabolism or excretion. Other illnesses may alter circulation, protein levels, receptor activity, or the physiological system affected by the drug.
Other drugs and substances: Interactions may change absorption, metabolism, transport, or drug effects. An enzyme inhibitor can increase exposure to a medicine that the enzyme normally breaks down, while an inducer can reduce exposure. Some interactions instead produce additive or opposing effects at the level of drug action.
Food, nutrition, and administration: Meals, supplements, route of administration, and the timing or consistency of doses can affect absorption or exposure. Follow medication-specific administration instructions.
and adherence: With repeated use, some effects may diminish, so a previously effective dose may have less effect. Missed doses, incorrect timing, or stopping treatment can also change response.
Individual sensitivity and expectations: Differences in target sensitivity, baseline physiology, and contextual or psychological factors can affect perceived or measured outcomes.
These influences can combine. For example, reduced kidney function and a drug interaction may both increase exposure, while a change in target sensitivity may alter the effect even when blood concentration is unchanged. Clinicians therefore assess the individual's response and adverse effects rather than relying on dose alone.