What a Drug Actually Is

TL;DR. A drug is a molecule that binds something in your body and changes what it does. Everything else about medicine follows from four processes (absorption, distribution, metabolism, excretion) and one idea (the dose-response curve). Two numbers explain most of what you need to know about any drug: its half-life, which tells you how often to take it and how long it lingers, and its therapeutic index, which tells you how much room there is between a dose that works and a dose that harms. Paracetamol has a narrow one; that is why it kills people. Understanding these makes every later chapter shorter.

1. What a drug does at the molecular level

Almost every drug works by binding to a specific protein and changing its behaviour.

Two terms recur through the rest of this book:

• Agonist: binds and activates, mimicking the natural signal. Morphine at opioid receptors.
• Antagonist: binds and blocks, preventing the natural signal. Naloxone at opioid receptors, which is why it reverses an overdose within minutes.

Selectivity is relative, not absolute. A drug that mainly hits one receptor also hits others at higher concentrations, and most side effects are the drug doing exactly what it does, in a tissue where you did not want it. Beta blockers slow the heart (intended) and can constrict airways (unintended), because the same receptor family appears in both places. Antihistamines block histamine in the nose (intended) and in the brain (sedation). Once you see this, side effects stop looking arbitrary.

2. ADME: the four things that happen to every drug

Absorption, Distribution, Metabolism, Excretion. Together they determine how much drug reaches the target, and for how long.

Absorption

Bioavailability is the fraction of a dose that reaches the systemic circulation unchanged. Intravenous is 100 percent by definition; everything else is less.

First-pass metabolism is the crucial oral concept. Everything absorbed from the gut goes through the portal vein to the liver before reaching the rest of the body (Chapter 10). The liver may destroy most of it on that first pass.

This explains a great deal:

• Oral morphine doses are 2 to 3 times the injected dose, because much is destroyed first.
• Glyceryl trinitrate is given under the tongue, because swallowed it is almost completely destroyed.
• Insulin cannot be given orally at all, because it is a protein and is digested.
• Liver disease can dramatically raise blood levels of drugs with high first-pass metabolism.

Distribution

Once in the blood, a drug spreads according to its chemistry:

• Water-soluble drugs stay largely in blood and extracellular fluid.
• Fat-soluble drugs accumulate in fat, which gives them long, unpredictable durations. This is why body composition changes dosing, and why some drugs behave differently in older people, who have proportionally more fat and less water.
• Protein binding: many drugs travel bound to albumin, and only the unbound fraction is active. A drug that is 99 percent bound has 1 percent doing the work, and anything that displaces it (another drug, low albumin from illness or malnutrition) can double the active concentration. This matters for warfarin and phenytoin in particular.
• The blood-brain barrier excludes most large and water-soluble molecules, which is why loratadine does not make you sleepy and chlorphenamine does.
• The placenta excludes very little. Assume a drug crosses unless told otherwise.

Metabolism

Mostly liver, in two phases:

• Phase I (mostly cytochrome P450 enzymes): oxidation, reduction, hydrolysis. Usually makes the drug more water-soluble, sometimes activates it, occasionally makes it more toxic.
• Phase II: conjugation, attaching glucuronide, sulphate, or glutathione, which makes the molecule water-soluble and easy to excrete.

The cytochrome P450 system is where most drug interactions happen, and three enzymes matter most:

• An inhibitor slows metabolism, so the drug accumulates and effects and toxicity increase.
• An inducer speeds metabolism, so the drug is cleared faster and stops working. This is why St John's wort can cause contraceptive failure and transplant rejection, and why quitting smoking can suddenly make a stable clozapine dose toxic.

Genetic variation matters enormously here. CYP2D6 in particular varies between people from non-functional to multiple extra copies:

• Poor metabolisers (5 to 10 percent of Europeans) get almost no effect from codeine, because codeine is a prodrug that must be converted to morphine by CYP2D6.
• Ultra-rapid metabolisers (up to 30 percent in some North African and Middle Eastern populations) convert it too fast and can reach dangerous morphine levels from a standard dose. This has killed children after tonsillectomy and breastfed infants of ultra-rapid metaboliser mothers, which is why codeine is now contraindicated in children under 12 and in breastfeeding in many countries.

Prodrugs are inactive until metabolised: codeine to morphine, enalapril to enalaprilat, clopidogrel activated by CYP2C19, levodopa to dopamine. If the activating enzyme is missing or blocked, the drug simply does not work.

Excretion

Mostly the kidneys, some in bile and faeces, small amounts in breath, sweat, and breast milk.

This is why kidney function dominates dosing. Reduced kidney function means reduced clearance means accumulation. Doses of many drugs (metformin, DOACs, gabapentin, many antibiotics, digoxin, lithium) must be reduced or the drug avoided in renal impairment, and eGFR is checked routinely for exactly this reason.

Kidney function declines with age even without disease, which is one reason older people need lower doses of many drugs (Chapter 83).

3. Half-life: the number that explains dosing

Half-life ($t_{1/2}$) is the time for the blood concentration to fall by half. It determines everything about scheduling.

Two rules follow, and they are the most useful arithmetic in pharmacology:

1. A drug is essentially gone after about 5 half-lives.
2. A drug reaches steady state after about 5 half-lives of regular dosing. Until then, levels are still climbing, which is why some medicines take days or weeks to work fully.

Loading doses exist to short-circuit rule 2: a large first dose fills the volume of distribution immediately, then maintenance doses hold it. This is why some antibiotic and anticoagulant courses start with a double dose.

4. Dose-response and the therapeutic index

More drug does more, up to a point, and then it does harm. Two curves matter: the one for the desired effect, and the one for toxicity.

The gap between them is the therapeutic index:

$$\mathrm{TI} = \frac{\text{dose that is toxic in 50% of the population}}{\text{dose that is effective in 50%}}$$

Paracetamol is the one to understand. Its maximum daily dose (4 g in most countries) and the dose at which liver damage begins (from around 7.5 to 10 g in a healthy adult, and lower in people who drink heavily, are malnourished, or are underweight) are uncomfortably close for a drug sold in supermarkets. That narrow gap is why it gets its own chapter (Chapter 65).

Efficacy versus potency, a distinction people mix up constantly:

• Potency is how much drug you need. A 5 mg drug is more potent than a 500 mg drug.
• Efficacy is how much effect it can produce at maximum.

Potency is nearly irrelevant to how good a drug is. Fentanyl is far more potent than paracetamol; that says nothing about which is better for a headache.

5. What actually determines your dose

This is why a book cannot give you your dose, and why the doses in later chapters are labelled illustrative.

6. Two more concepts you will meet

Tolerance. With repeated exposure, the same dose produces less effect, through receptor downregulation, enzyme induction, or compensatory changes. Prominent with opioids, benzodiazepines, nitrates, alcohol, and caffeine. Cross-tolerance means tolerance to one drug confers tolerance to related ones.

Dependence and withdrawal. The body adapts to a drug's presence and functions abnormally when it is removed. This is physiological and is not the same as addiction, which is a behavioural condition involving compulsive use despite harm. People become physically dependent on antidepressants, beta blockers, corticosteroids, and proton pump inhibitors without being addicted to them. Conflating the two causes real harm in both directions. It is why several drug classes must be tapered rather than stopped abruptly: benzodiazepines and alcohol (seizure risk), opioids, beta blockers (rebound tachycardia and angina), corticosteroids (adrenal crisis), and SSRIs (discontinuation syndrome).

Placebo and nocebo. Placebo responses are real and measurable, mediated by expectation, conditioning, and endogenous opioid and dopamine release, and they are largest for subjective outcomes like pain, nausea, and mood. Nocebo is the mirror: expecting side effects produces them. In statin trials, the majority of reported muscle symptoms occurred equally on placebo, and n-of-1 trials have demonstrated this in individual patients (Chapter 85).

7. The bottom line

• A drug binds a receptor, enzyme, channel, or transporter and changes what it does. Most side effects are the same action in the wrong tissue.
• ADME determines exposure: absorption, distribution, metabolism, excretion. Oral drugs pass through the liver first, which is why oral and injected doses differ so much.
• The cytochrome P450 system, especially CYP3A4, is where most interactions live. Inhibitors make drugs accumulate; inducers make them stop working.
• Half-life gives you two rules: essentially gone after five half-lives, and at steady state after five half-lives of regular dosing.
• The therapeutic index is the gap between working and harming. Warfarin, lithium, digoxin, and paracetamol are the ones where that gap is small.
• Physical dependence is not addiction, and several drug classes must be tapered rather than stopped.

Sources and notes

Pharmacokinetic and pharmacodynamic principles follow Rang and Dale's Pharmacology and Katzung's Basic and Clinical Pharmacology. Bioavailability by route, first-pass metabolism, and protein binding follow those texts and the BNF. Cytochrome P450 substrate, inhibitor, and inducer tables follow the FDA's drug interaction guidance and the Indiana University P450 interaction table. CYP2D6 polymorphism frequencies and the codeine ultra-rapid metaboliser deaths follow the CPIC guideline for codeine and the FDA and EMA safety reviews that led to the paediatric contraindication. Half-life arithmetic and steady state follow standard pharmacokinetics. Narrow therapeutic index drug lists follow FDA and MHRA classifications. Nocebo neurobiology follows Benedetti's work. The distinction between physical dependence and addiction follows DSM-5 and the Royal College of Psychiatrists' position statements.

Open questions. Pharmacogenomic testing is technically available for CYP2D6 and CYP2C19 and is not routine in most health systems, so most prescribing still proceeds without knowing the patient's metaboliser status.

👉 Next: how a drug is made, tested, and approved.