1 Foundations of Pharmacology

Pharmacology and drug classifications

Pharmacology is the study of how drugs interact with living systems. A drug may prevent, diagnose, relieve, or treat a condition, but its clinical effect depends on both the drug and the person receiving it.

Two connected areas help explain medication therapy:

• Pharmacokinetics describes what the body does to a drug: its movement into, through, and out of the body.

• Pharmacodynamics describes what a drug does to the body: its mechanisms and resulting effects.

A medication can have several classifications at once. Its therapeutic class describes the condition or purpose it addresses; its pharmacologic class describes how it works. Regulatory status, such as prescription or over-the-counter, concerns how a medicine may be obtained or regulated rather than its mechanism. Knowing a drug’s class can help anticipate intended effects and possible risks, but drugs in the same class are not necessarily interchangeable in every situation.

FDA-approved generic medicines are required to provide the same clinical benefit and risks as their brand-name counterparts.

Pharmacokinetics: the ADME processes

Pharmacokinetics is commonly organized as ADME: absorption, distribution, metabolism, and excretion. Together, these processes influence how much drug reaches its site of action and when effects begin and end.

Absorption and bioavailability

Absorption is the movement of a drug from its administration site into the bloodstream. Route, formulation, blood flow, and other conditions can affect it. An intravenous drug enters systemic circulation directly, so it does not need to be absorbed from the gastrointestinal tract. An oral drug may be partly broken down in the gut or liver before reaching systemic circulation; this is the first-pass effect.

Bioavailability is the fraction of an administered dose that reaches systemic circulation as active drug. It is generally highest with intravenous administration.

Distribution

Distribution is the movement of a drug between blood and tissues. Blood flow, tissue permeability, body composition, and protein binding can affect it. The unbound portion is generally available to leave the bloodstream and interact with targets; protein-bound drug may act as a temporary reservoir.

Metabolism

Metabolism is the chemical alteration of a drug, often by enzymes in the liver. It may inactivate a drug, activate a prodrug, or produce active or inactive metabolites. Illness, age, genetics, and interacting substances can change metabolism.

Excretion, half-life, and steady state

Excretion removes a drug or its metabolites from the body. Many drugs are cleared primarily through the kidneys. Reduced kidney or liver function can slow clearance and allow a drug to accumulate.

Half-life is the time required for a drug’s concentration in the body to fall by about half under the relevant elimination conditions. It helps inform dosing intervals and the time needed to approach steady state. Steady state occurs during repeated dosing when the rate of drug administration and the rate of elimination are balanced.

Takeaway: ADME links the route of administration and the body’s handling of a drug to the concentration and timing of its effects.

Pharmacodynamics: targets and effects

Drugs can produce effects by interacting with receptors, enzymes, ion channels, transporters, or other biological targets. A drug’s mechanism of action describes how it produces an effect.

At a receptor, an agonist binds and activates it. An antagonist binds and blocks activation by another substance. A partial agonist activates a receptor but produces a smaller response than a full agonist, even when it occupies many receptors. For example, many beta-blockers block beta-adrenergic receptors.

A dose–response relationship describes how a drug’s effect changes as its dose or concentration changes. Potency is the amount of drug needed to produce a specified effect; efficacy is the maximum effect the drug can produce. Greater potency does not necessarily mean greater clinical benefit.

The therapeutic window is the range between concentrations likely to be effective and those likely to cause toxicity. A narrow therapeutic index means this range is relatively small, so dose changes and monitoring may require particular care.

Takeaway: The way a drug binds to a target, and the size of the response it can produce, are distinct aspects of its effects.

Why medication responses differ

People can respond differently to the same drug and dose because individual factors may affect pharmacokinetics, pharmacodynamics, or both. Important influences include:

• Age and development: Infants and older adults may process or respond to some medicines differently.

• Body size and composition: Weight, body fat, and body water can affect distribution and dosing.

• Kidney or liver function: Impaired elimination or metabolism may increase drug exposure.

• Genetics: Inherited differences can alter drug-metabolizing enzymes, transporters, or targets.

• Other medicines, supplements, food, and beverages: These may change absorption, metabolism, or effects, creating interactions.

• Disease, hydration, and nutritional status: Illness and physiological changes can alter drug handling or sensitivity.

• Tolerance and adherence: Repeated exposure may reduce response to some drugs; missed doses or incorrect use can also change treatment outcomes.

• Route, formulation, and storage: These affect the amount and rate of drug reaching the body and whether the product remains usable.

For example, if kidney function declines, a drug normally cleared by the kidneys may remain in the body longer. The prescribed regimen may therefore need reassessment by the treating clinician. Medication response is individualized rather than reliably predicted by a standard dose alone.

Takeaway: Safe and effective medication use depends on the drug and the person’s circumstances, not just on a drug’s classification or a standard dose.