Overview
Pharmacology is the science of how drugs interact with living systems. A drug is any chemical that changes a biological function. To use drugs safely we ask two paired questions: What does the body do to the drug? — this is pharmacokinetics — and What does the drug do to the body? — this is pharmacodynamics. Everything in later lessons about routes, drug classes, and dosing rests on these two ideas.
Pharmacokinetics: The ADME Journey
Pharmacokinetics tracks a drug’s concentration as the body processes it. The four stages spell ADME.
| Stage | What happens | Key influences |
|---|---|---|
| Absorption | Drug enters the bloodstream from its site of administration | Route, drug form, dissolving rate |
| Distribution | Drug travels in blood and moves into tissues | Blood flow, protein binding, fat solubility |
| Metabolism | Drug is chemically altered, usually to an inactive, water-soluble form | Mainly the liver (cytochrome P450 enzymes) |
| Excretion | Drug and its byproducts leave the body | Mainly the kidneys (urine); also bile, sweat, breath |
Two related concepts matter clinically. Bioavailability is the fraction of a dose that actually reaches the systemic circulation in active form. An intravenous dose is 100% bioavailable; an oral dose is often less because of the first-pass effect — the liver metabolizes some of the drug after it is absorbed from the gut but before it reaches the rest of the body.
Half-life is the time for the blood concentration to drop by half. It determines how often a drug must be dosed and how long effects last. After about four to five half-lives, a drug is largely eliminated, and with repeated dosing it reaches a stable steady state.
Distribution deserves a closer look. Many drugs travel through the blood partly bound to plasma proteins such as albumin. Only the free (unbound) fraction can leave the vessels and act on tissues, so protein binding acts as a reservoir that releases drug slowly. Fat-soluble drugs cross cell membranes and the blood–brain barrier easily and may accumulate in fatty tissue, while water-soluble drugs stay mostly in the blood and body fluids. These properties help explain why two drugs given at the same dose can reach very different tissues.
Pharmacodynamics: How Drugs Act
Most drugs work by binding to a receptor — typically a protein — and changing the activity of a cell. The fit between drug and receptor is often compared to a key in a lock.
- An agonist turns the receptor on, imitating a natural messenger (for example, a drug that mimics adrenaline).
- An antagonist blocks the receptor, so the natural messenger cannot act (for example, a “beta-blocker” that shields the heart from adrenaline).
- A partial agonist activates the receptor only weakly.
Not all drugs use receptors. Some work through enzymes, ion channels, or simple chemistry (an antacid, for instance, neutralizes stomach acid directly).
Dose–Response and Safety
As dose rises, effect increases until receptors saturate and the curve plateaus — the dose–response relationship. Potency describes how much drug is needed for an effect; efficacy describes the maximum effect possible.
The therapeutic index compares the toxic dose to the effective dose. A wide index (large gap) is forgiving; a narrow index means the helpful dose sits dangerously close to the harmful dose, so clinicians monitor blood levels closely.
Side Effects and Interactions
No drug is perfectly selective. Side effects are unwanted effects at normal doses; adverse effects are harmful reactions; an allergy is an immune reaction to a drug. Drug interactions occur when one drug changes another’s action — for example, by competing for the same metabolizing enzyme and raising a second drug’s levels toward toxicity.
Response to a drug also varies from person to person. Age matters: newborns and older adults often have slower metabolism and excretion, so standard doses can build up. Genetics can make one person metabolize a drug quickly and another slowly. Body weight, organ function (especially of the liver and kidneys), other medications, and even food can all shift how a drug behaves. Because of this variability, doses are frequently individualized and, for narrow-index drugs, guided by blood-level monitoring.
Tolerance and Dependence
With repeated exposure, the body can adapt to some drugs. Tolerance means a larger dose is needed over time to achieve the same effect. Dependence means the body has adjusted so that stopping the drug abruptly produces withdrawal symptoms. Both are pharmacodynamic phenomena that clinicians anticipate when starting and stopping certain medications.
Clinical Relevance
Understanding these principles explains everyday clinical decisions. A patient with reduced kidney function may accumulate a drug that is cleared by the kidneys, so the dose is lowered. A drug with a short half-life must be taken several times a day to stay effective. A narrow-therapeutic-index drug is dosed carefully and its blood levels are checked. Recognizing that the liver and kidneys govern metabolism and excretion helps caregivers anticipate why the very young, the elderly, and those with organ disease often respond differently to the same medication.