1 Foundations of Pharmacology and Therapeutic Effects
Learn how drugs move through the body, produce therapeutic and unintended effects, and vary in their effects across people.
Two perspectives on drug effects
is the study of how drugs interact with living systems. Two complementary perspectives explain medication effects: describes what the body does to a drug, while describes what the drug does to the body. A medication’s effect depends both on how much drug reaches its site of action and on how that site responds.
How the body handles drugs
The four linked processes of are absorption, distribution, , and excretion, together called ADME. These processes influence a drug’s concentration over time.
Absorption and
Absorption is movement from the administration site into systemic circulation. Route and formulation matter: an oral tablet must dissolve and pass through the gut wall, whereas an intravenous dose enters systemic circulation directly.
Oral drugs may undergo first-pass in the intestine and liver before reaching general circulation, reducing the amount available. is the fraction of a dose that reaches systemic circulation unchanged.
Distribution
Distribution is movement of a drug from blood into tissues and fluids. Blood flow, tissue barriers, body composition, and binding to plasma proteins affect distribution. In general, unbound drug is more readily available to enter tissues and interact with targets.
, also called biotransformation, chemically changes a drug, often in the liver. The resulting metabolites may be inactive, active, or—in some cases—harmful. Some medicines are prodrugs: converts them into an active form.
Excretion and clearance
Excretion removes drugs and metabolites from the body. The kidneys are a major route; drugs may also leave through bile and feces or, for some substances, through the lungs. Reduced kidney or liver function can alter drug clearance and exposure.
For example, if kidney function declines, a drug normally cleared in urine may remain in the body longer. Its concentration can rise with repeated doses, increasing the chance of unwanted effects unless the treatment plan accounts for the change.
How drugs produce effects
A drug can produce an effect by interacting with a biological target, such as a receptor, enzyme, ion channel, or transporter. An activates a receptor; an binds and blocks or reduces activation by another substance. Drugs can also change enzyme activity or alter the movement of substances across cell membranes.
The is the intended clinical benefit, such as pain relief from an analgesic. A drug can also produce side effects or other unintended effects. An is a harmful or unwanted response at usual doses, and effects vary in seriousness. An allergic reaction is an immune response and is distinct from predictable effects caused by a drug’s .
Dose, effect, and safety
and describe different aspects of a drug’s dose-response relationship. describes how much drug is needed to produce an effect; describes the maximum effect the drug can produce. A drug that is more potent is not necessarily more effective or safer.
The is the range between concentrations likely to be effective and those likely to be toxic. A narrow means careful dosing and, for some drugs, monitoring are especially important.
Why medication responses differ
The same dose can have different effects in different people. Responses may reflect several influences acting together:
Patient characteristics: age, body size and composition, pregnancy, and genetic variation.
Organ function and health: kidney or liver impairment, blood flow, and other illnesses can change drug handling or sensitivity.
Route, formulation, and use: route of administration, food, timing, and taking a medicine as directed can change absorption or exposure.
Other substances: prescription and nonprescription medicines, supplements, alcohol, and some foods may alter absorption, , drug concentrations, or combined effects. Enzyme inhibitors can slow some metabolic pathways, while inducers can speed them up; the outcome depends on the drugs involved.
Biological variation: genetic differences affecting drug-metabolizing enzymes, transporters, or drug targets can change exposure or response. Genetic information is only one factor and does not, by itself, predict every person’s response.
Because these influences can combine, medication decisions rely on the specific drug, the person’s circumstances, and appropriate clinical monitoring—not on dose alone.