Drug Names, Lab Values, and Medical Words, Decoded

TL;DR. Medical language looks like memorisation and is mostly assembly. Drug names are built to a published international standard in which the ending tells you the class: anything ending -statin lowers cholesterol by blocking the same enzyme, anything ending -pril blocks the same blood pressure enzyme, anything ending -mab is an engineered antibody. Disease names are built from a small set of Greek and Latin parts: -itis is inflammation, -osis is a process or state, -aemia is in the blood, hyper- and hypo- are too much and too little. Learn about forty pieces and you can make a reasonable guess at the meaning of a term you have never seen, which is the difference between reading a leaflet and being read to.

Key takeaways

• Every drug has three names: a chemical name nobody uses, a generic name that is the same worldwide, and one or more brand names that differ by country and company.
• The generic name's ending (the stem) identifies the class, and it is assigned by the WHO under the International Nonproprietary Name system, so it is reliable.
• Ozempic and Wegovy are both semaglutide. Brand names hide the fact that two products are the same molecule; generic names reveal it.
• Agonist means it activates, antagonist or blocker means it prevents activation, inhibitor means it stops an enzyme or pump. Almost every drug is one of these three.
• Lab values come in two unit systems, and the same result is written 126 mg/dL in the United States and 7.0 mmol/L almost everywhere else.
• Leaflets use defined frequency words: "common" means between 1 in 100 and 1 in 10, not "often."

Part 1: How drugs are named

In short: The ending of a generic name tells you the drug class, because the WHO assigns those endings deliberately.

The three names

Take the common cholesterol drug:

Always find the generic name. It is the same in every country, it tells you the class through its ending, and it is the only way to know whether two products are the same drug. It is usually printed under the brand name on the packet.

The stem system: the ending tells you the class

The WHO assigns each new generic name an ending, called a stem, shared by every drug that works the same way. This is a deliberate international scheme, not a coincidence, and it is the single most useful thing a non-specialist can learn about medicines.

Heart, circulation, and metabolism

Infection

Cancer and immunity

Brain, gut, and the rest

How to decode a name you have never seen

Work from the end backwards.

• Empagliflozin. Ends in -gliflozin → an SGLT2 inhibitor → blocks the kidney's glucose pump → used in diabetes, heart failure, and kidney disease → expect glucose in the urine and therefore genital yeast infections.
• Osimertinib. Ends in -tinib → a kinase inhibitor → a targeted cancer drug blocking a signalling enzyme → so it needs a tumour with that specific mutation to work.
• Esomeprazole. Ends in -prazole → a proton pump inhibitor → reduces stomach acid. The "es-" prefix means it is the purified single mirror-image form of omeprazole.
• Sacubitril/valsartan. Two drugs in one tablet. The second ends in -sartan → an angiotensin receptor blocker. The first is a neprilysin inhibitor. This is the heart failure drug called an ARNI in Chapter 21.
• Benralizumab. Ends in -mab → an engineered antibody. From the table below, the -li- before it signals an immune target. Used in severe eosinophilic asthma.

Reading an antibody name in detail

Until 2021, monoclonal antibody names were built in four parts, and old names still follow it:

[unique prefix] + [target] + [source] + mab

So trastuzumab = tras + tu (tumour) + zu (humanised) + mab: a humanised antibody against a tumour target, which is HER2 in breast cancer. Infliximab = infli + xi (chimeric) + mab. Adalimumab = ada + limu (immune, human) + mab.

The source letters matter clinically: the more mouse protein an antibody contains, the more likely the patient's immune system is to develop antibodies against the drug itself, reducing its effect over time.

In 2021 the WHO replaced the single -mab ending with four, because antibody engineering outgrew the old scheme: -tug (an unmodified antibody), -bart (an artificial one), -mig (bispecific, gripping two targets at once), and -ment (an antibody fragment). New drugs use these; the thousands of existing -mab names remain.

The action words

Almost every drug does one of a small number of things, and the words are used precisely.

Two of these are worth pausing on because they explain a lot of side effects. A receptor usually exists in more than one tissue, so an agonist or blocker will act everywhere that receptor is found, wanted or not (Chapter 16). And an enzyme usually has more than one job, so an inhibitor blocks all of them.

Part 2: Building and reading medical words

In short: Learn about forty prefixes, roots, and suffixes and you can decode most terms you have never seen.

Almost every medical term is a compound of a prefix (how much, where, what kind), a root (the body part), and a suffix (what is happening to it).

Suffixes: what is happening

Prefixes: how much, where, and what kind

Body-part roots

Worked examples, assembling from the parts:

• Pericarditis = peri (around) + cardi (heart) + itis (inflammation) = inflammation of the sac around the heart.
• Thrombocytopenia = thrombo (clot) + cyt (cell) + penia (deficiency) = too few platelets, the cells that make clots.
• Hepatosplenomegaly = hepato (liver) + spleno (spleen) + megaly (enlargement).
• Nephrolithiasis = nephro (kidney) + lith (stone) + iasis (condition) = kidney stones.
• Cholecystectomy = chole (bile) + cyst (bladder) + ectomy (removal) = gallbladder removal.
• Bronchopneumonia = broncho (airways) + pneumon (lung) + ia = infection centred on the small airways and surrounding lung.

Part 3: Lab values and units

In short: The same blood result is written two different ways depending on the country, and the conversions are worth having.

Two unit systems for the same thing

The United States generally uses conventional units (mg/dL); most of the rest of the world uses SI units (mmol/L). The same blood result is written differently, which makes international reading confusing and occasionally dangerous.

HbA1c has two systems of its own. The older DCCT/NGSP system reports a percentage; the IFCC system reports mmol/mol. Many labs now print both.

Other units you will meet

• mmHg (millimetres of mercury): blood pressure, inherited from mercury column instruments. 120/80 means the peak and trough pressures could each support a column of mercury that many millimetres high.
• IU (international unit): used where a substance is measured by biological activity rather than by weight, because the pure substance is hard to weigh consistently. Insulin, heparin, and several vitamins are dosed this way. One IU of insulin is defined by its glucose-lowering effect, not by its mass.
• eGFR, in mL/min/1.73m²: estimated kidney filtration rate, standardised to an average adult body surface area so that people of different sizes can be compared.
• Micrograms (mcg or µg) versus milligrams (mg): a thousand-fold difference and a recurring source of fatal error. Written prescriptions increasingly spell out "microgram" rather than abbreviating it, for exactly this reason.

Prescription shorthand

Part 4: Reading a package leaflet

In short: The frequency words are defined numbers rather than ordinary English, and a long side effect list is a legal document rather than a danger signal.

The frequency words are defined, not descriptive

European and UK leaflets use standard categories, and most people read them as ordinary English rather than as the specific numbers they are:

So a "common" side effect happens to at most one person in ten, and a "very rare" one to fewer than one in ten thousand. A leaflet listing thirty side effects is not describing a dangerous drug; it is a legal document listing everything ever reported, including things that happened at the same rate in the placebo group.

What the sections mean

• Contraindications ("do not take if"): situations where the drug should not be used at all, because the risk clearly exceeds the benefit.
• Warnings and precautions: situations needing caution, monitoring, or dose change.
• Interactions: other drugs, and sometimes foods, that change this drug's level or effect, most often through the liver enzymes described in Chapter 16.
• Black box or boxed warning (US) and equivalent notices elsewhere: the strongest warning a regulator applies, indicating a serious or life-threatening risk.

Reading the numbers in a study or a news report

• "Doubles the risk" is meaningless without the starting risk. Ask what the risk was before.
• Absolute risk (2 in 100 became 1 in 100) is the useful number. Relative risk (a 50 percent reduction) is the impressive one, and describes the same event.
• Number needed to treat answers "how many people take this for one to benefit."
• Confidence interval: the range within which the true value probably lies. A wide interval that crosses "no effect" means the study could not tell.
• Statistically significant means unlikely to be chance. It does not mean large, and it does not mean it matters to a patient.

Chapter 17 covers this properly, and these five points handle most headlines.

Part 5: Words people commonly mix up

In short: Sixteen pairs that are used interchangeably in conversation and mean different things in medicine.

Sources and notes

Drug naming stems are defined by the WHO INN programme, whose published stem list is the authoritative source; the examples here follow it. The monoclonal antibody nomenclature revision replacing -mab with -tug, -bart, -mig, and -ment was adopted by the WHO INN Expert Group in 2021. Unit conversion factors are standard clinical chemistry. HbA1c DCCT-to-IFCC conversion follows the published master equation. Adverse reaction frequency categories are the European Commission's guideline on Summary of Product Characteristics, used in EU and UK patient information leaflets. Medical word roots follow standard medical terminology references.

A caution. Stems are reliable and not absolute: a few older drugs predate the system and some names are historical accidents. Salbutamol, for instance, is a beta-2 agonist without the -terol ending its relatives carry, and paracetamol/acetaminophen has two entirely different generic names in different countries, which is one of the most consequential naming problems in medicine because people take both without realising they are the same drug and exceed the safe dose.

That last example is the argument for this whole chapter. 🎓