A cup of coffee gives you a lift within half an hour. Two cups give a stronger one, and ten give you the shakes. Have one at night and you may still be awake at midnight, because the caffeine is still in your blood, and the person next to you, who drank the same, is asleep. Four ideas are hiding in that one cup: the effect depends on the dose; there is a ceiling and a downside; the drug leaves the body at a particular speed; and bodies differ.
Those four ideas are pharmacology. Four hundred years before it had a name, the physician Paracelsus wrote that the dose, not the substance, decides whether something is a poison. This piece explains how medicine works and is not medical advice. It gives no doses of any real medicine: the numbers in the interactives are for a made-up "drug X" in made-up units. For a health question, see a clinician or pharmacist; in an emergency, call your local emergency number.
Most drugs work by binding to a protein, usually a receptor (a lock on the surface of a cell that the body's own messengers, such as adrenaline, normally open). The idea that cells carry "receptive substances" for specific chemicals dates to the early 1900s. A drug that fits and switches the lock on is an agonist. The more drug there is, the more receptors are occupied, until they are all busy and more drug does nothing: a ceiling (the maximum effect, Emax).
On an ordinary scale this curve bends flat; on a log scale it becomes an S-shape, and that is how pharmacologists always draw it. Two numbers define a drug: potency (how far left the S sits, set by EC50) and efficacy (how high it goes). They are not the same: a potent drug is simply effective at a small dose, not a stronger one.
The occupancy-style (Hill) equation with a made-up drug X in made-up units. No real medicine is represented.
For simple binding (n = 1) the dose must rise 81-fold to carry the effect from 10% to 90%, which is why a log axis is needed. A steeper drug (n = 2) covers that span in a 9-fold change, so a small error in dose matters more.
A molecule can fit the lock without turning it. An antagonist binds to the receptor and does nothing except block the way. This is a profound idea: much of modern medicine works by blocking. The Scottish pharmacologist James Black built propranolol, which blocks adrenaline's beta receptors on the heart, and cimetidine, which blocks histamine receptors in the stomach; he shared the 1988 Nobel Prize in Physiology or Medicine for this "rational" design. The best-known rescue example is naloxone, which sits on opioid receptors and displaces opioids such as morphine.
There are four useful cases. A competitive antagonist fights the agonist for the same lock; flood the system with agonist and you can still win, so the curve slides right but the ceiling is unchanged. The shift is predictable: the dose ratio = 1 + [antagonist] ÷ its binding constant (the Schild relation, after Heinz Otto Schild). A non-competitive (insurmountable) blocker lowers the ceiling itself. A partial agonist turns the lock only part-way, so its ceiling is low, however much you give; it also stops a full agonist from working fully, which is exactly how buprenorphine is used with opioids.
Standard textbook models (Schild shift; reduced ceiling; fixed 50% partial agonist). The partial agonist chip ignores the slider because its ceiling is fixed at 50%. Made-up drugs.
With a competitive blocker at five times its binding constant (slider at 10), the agonist needs 6 times the dose for the same effect. At 20 times (slider at 40) it needs 21 times. The shift is the signature of competition.
Every effective drug is also harmful at a high enough dose, so what matters is the gap. Doctors speak of the therapeutic index: the dose that causes harm in half of people divided by the dose that works in half of people. A wide index (say, 100) gives room for error. A narrow one (say, 2 or 3) means the working dose and the harmful one overlap in the population, and treatment needs close monitoring, often with blood tests. Familiar narrow-index medicines include the blood thinner warfarin, the heart drug digoxin and lithium. They are not unsafe; they are respected.
Two idealised population curves of equal steepness (n = 3), as a teaching picture. Real drugs have messier curves, and harm is often more than one effect.
With an index of 10 and a dose three times the median effective dose, about 97% benefit and 3% are harmed. A stricter measure compares the dose that works in 99% with the dose that harms 1%; at an index of 10 those curves overlap (the ratio is 0.47), and it takes an index near 100 to open a gap (4.7). A narrow index is a design constraint on every prescription.
A drug that works in a test tube still has to arrive. Pharmacokinetics tracks four steps, remembered as ADME: absorption (into the blood, often through the gut wall), distribution (out of the blood into tissues), metabolism (usually chemically changed by enzymes, mostly in the liver) and excretion (usually by the kidneys, into urine). A pill swallowed goes first through the liver on the way to the rest of the body, where part is destroyed: the "first-pass effect", one reason oral and injected versions of a drug need different amounts.
The one quantity worth keeping is the half-life: the time for the level to fall by half. After one half-life 50% is left, after two 25%, and so on. Most drugs clear in proportion to what is there, so the level follows an exponential, and the same arithmetic gives the schedule. In terms of numbers: half-life = 0.693 × V ÷ CL, where V is the volume the drug seems to spread through and CL is clearance, the volume of blood cleared of the drug per hour.
Here is a fictional drug X: a 400-unit dose into a 40-litre volume gives 10 units per litre at the start. Change the half-life and dosing and watch the curve. With a half-life of 8 hours and a dose every 12 hours, the highest level climbs from 10 to about 15.4 units/L as the doses overlap, an accumulation factor of 1.55; it takes about 27 hours (3.3 half-lives) to reach 90% of its final level. A swallowed dose rises more slowly, peaks lower (about 6.3 for a single dose, with 80% of it getting in) and falls with the same half-life.
Fictional drug X: 400 units per dose, 40 L volume, swallowed form absorbed with rate 1 per hour and 80% reaching the blood. Round numbers for practice. Not a model of any real medicine and not a dosing tool.
Real people take more than one drug and differ in the enzymes that clear them. Some of the most important differences are in the cytochrome P450 liver enzymes. A slow metabolizer clears a drug slowly, so ordinary doses pile up; an ultrarapid one clears it fast, so the same dose may do too little. When the drug is a prodrug (it must be changed in the body to become active), the effect is reversed. Codeine, converted by the enzyme CYP2D6 to morphine, is the standard example, and regulators have restricted it in children partly because of fast converters. Frequencies differ by ancestry: reviews report that roughly 7% to 10% of people of European ancestry have reduced CYP2D6 function, with a smaller share in many other groups.
Other kinds of variation are about the immune system, not clearance. An HLA gene variant, HLA-B*15:02, that is more common in some East and Southeast Asian ancestries, is linked to a rare, severe skin reaction to the epilepsy drug carbamazepine, so prescribing information advises considering a genetic test before it is started in people of those ancestries. Interactions work the same way: an inhibitor of an enzyme acts like a slow metabolizer, and an inducer like a fast one. Grapefruit juice blocks an enzyme in the gut wall and can raise levels of some blood-pressure drugs. Age, kidney and liver function, pregnancy and body size all move clearance as well.
Drug X again: 400 units every 12 hours, volume 40 L, typical clearance 4 L/h. Clearance multipliers 1, 0.4, 2.5 and 0.5 are illustrative. The "useful window" is made up. Average level = dose ÷ (clearance × interval).
Clearance changes from 4 to 1.6 L/h and the same schedule goes from 8.3 to 20.8 units/L, above the window; at 10 L/h it falls to 3.3, below it. Not every drug is proportional, either. When the enzymes are close to full, an extra dose is cleared no faster, and the level climbs out of proportion (alcohol and the epilepsy drug phenytoin are textbook cases).
Michaelis–Menten clearance for drug X (maximum 40 units/h, half-saturation 5 units/L), compared with a proportional model that matches it at low levels. Made-up numbers.
Double the dose rate from 10 to 20 units/h and the proportional model doubles the level (1.25 to 2.5). The saturable one triples it (1.7 to 5.0); at 30 it is 15, at 39 it is 195, and at 40 there is no steady level at all.
A candidate drug passes through preclinical work (cells, then animals) and then four phases in people. Phase 1: a few dozen volunteers, to see how it is handled and whether it is tolerated. Phase 2: a few hundred patients, to see whether it seems to work and at what dose. Phase 3: hundreds to thousands, usually randomised and often blinded, against a placebo or the current treatment. Then regulators (the US FDA, the European Medicines Agency, the UK's MHRA, Japan's PMDA, China's NMPA, Brazil's ANVISA, Mexico's COFEPRIS and others) weigh the evidence. Phase 4 and surveillance continue after approval, because rare harms only show up in large populations. The whole process often takes a decade or more.
The system was built from disasters. In 1937 the US elixir sulfanilamide, a medicine dissolved in a poisonous solvent, killed about 107 people and led to the 1938 Food, Drug, and Cosmetic Act. Thalidomide, sold in dozens of countries as a sedative and for morning sickness from the late 1950s, caused serious birth defects in more than 10,000 children before Widukind Lenz linked it to them in 1961. In the US, the reviewer Frances Kelsey had held up its approval, and the 1962 Kefauver–Harris Amendment required that drugs be shown to be safe and effective. Every phase is, in effect, an answer to something that once went wrong.
Most candidates fail. One analysis of drugs in development between 2003 and 2011 (Hay and colleagues, 2014) found that only about 10% of those entering clinical trials were approved. Failure is mostly for lack of benefit or unexpected harm, which is why a trial is a test and not a formality. Drag the survival rate of each stage below; these are round numbers close to that picture, not the paper's exact figures.
Illustrative round stage rates (65%, 30%, 58%, 90%) multiplying to about 10%. Real rates differ by disease area, era and company.
At those defaults, 1,000 candidates become about 102 approvals. Notice where the loss is: the second phase, where the first honest test of benefit happens, takes the most.
A drug is a key for a lock, and its effect is an S-curve with a ceiling. Others can block, partly turn or lower the ceiling; safety is the gap between good and harm; the body clears the drug at a rate that sets its schedule; people differ; and the whole thing is tested because most candidates fail. The right question about any medicine is never just "does it work?" but "how much, in whom, for how long, and with what gap before harm?"
Try this: next time you read a medicine's information leaflet, find three things it tells you: what the drug does (the target), how often it is taken (the half-life), and which other drugs or foods to avoid (the interactions). Do not change anything; just read it as a pharmacologist would. Ask your pharmacist if anything is unclear.
Sources. The Hill/Emax model, Schild relation, therapeutic index, one-compartment pharmacokinetics and Michaelis–Menten clearance are standard textbook pharmacology (Rang & Dale; Katzung). James Black and the 1988 Nobel Prize, the Schild relation, CYP2D6 variation, the HLA-B*15:02 and carbamazepine link, grapefruit interactions, Paracelsus's dose principle, clinical-trial phases, elixir sulfanilamide (1937; 107 deaths; 1938 Act), thalidomide (46 countries; more than 10,000 children; Lenz 1961; Kelsey; Kefauver–Harris 1962) were checked against encyclopaedia articles and their sources. Hay et al., Nature Biotechnology 32:40–51 (2014) for the approximate 10% overall success rate (checked through a secondary summary; the individual stage rates in the interactive are round, illustrative numbers). Drug X is fictional; all interactive numbers were computed in node. Not medical advice.