Units on a veterinary medicine label: IU, mg/mL, percent, CFU and titre
A percentage, a milligram per millilitre and an international unit are three different kinds of statement. Here is what each one measures, which convert into each other and which never do.
The short answer
- An international unit (IU) measures biological activity against an agreed reference preparation, not mass. There is no universal IU-to-milligram conversion — the factor is specific to each substance.
- In a weight-in-volume solution, 1% is 10 mg/mL, so percentage × 10 gives mg/mL. Confirm the label uses w/v before applying it: ointments and premixes are usually w/w.
- CFU counts viable micro-organisms in a live product; titre expresses infectivity or activity from a specific assay, often as a log10 value. The two are not interchangeable.
- Concentration describes the product; volume describes the packaging. A 250 mL bottle at 50 mg/mL contains less active substance than a 100 mL bottle at 200 mg/mL.
- The authoritative statement of strength is the 'qualitative and quantitative composition' section of the approved label, not the front of the carton.
A veterinary label states strength in whichever unit the product’s chemistry makes measurable, and it does not translate between them for you. On one shelf a bottle says 10%, the next says 100 mg/mL, a third says 1 500 IU per millilitre, and a sachet says 10⁹ CFU per gram. Three of those four describe an amount; one describes an effect. Knowing which is which is the difference between a comparison that means something and one that only looks like it does.
This page explains what each unit measures, which of them convert into each other, and where on an approved label the authoritative figure is written.
What the numbers on a veterinary label actually measure
The strength of a veterinary medicine is the quantity of active substance contained in a defined quantity of product: per millilitre, per gram, per tablet, or per dose. Everything else on the container — the volume, the pack size, the number of tablets — describes the packaging rather than the medicine.
That single sentence resolves most confusion, because the unit chosen depends on what can actually be measured. A chemically defined molecule can be weighed, so its strength is a mass per volume or a mass per mass. A substance whose effect cannot be reliably predicted from its weight is measured by what it does in a standardised test, and that gives a unit of activity rather than mass. A product containing living organisms is measured by counting the ones that are still alive.
| Unit | What it measures | Typically found on |
|---|---|---|
| mg/mL, mg/g, mg per tablet | Mass of active substance per unit of product | Injectable and oral solutions, tablets, ointments, premixes |
| Percentage (% w/v or % w/w) | The same mass, written as a proportion | Solutions, ointments, dips, in-feed premixes |
| IU (international unit) | Biological activity against a reference preparation | Vitamins A, D and E, some hormones and antibiotics |
| CFU, TCID50, EID50, PFU, titre | Viable organisms or infectivity, per dose or per gram | Vaccines, probiotics, live microbial feed additives |
What does IU mean on a veterinary product?
An international unit is a quantity of biological activity, defined by agreement against a reference preparation held by an international standards body. It is not a mass, and it is not the same size for two different substances.
The reason the unit exists is practical. For several substances, the biological effect of a given weight varies with the exact chemical form, the isomer, the salt or the ester used. Vitamin E is the standard illustration: several tocopherol forms carry different activity per milligram, so a manufacturer that stated only the milligrams would not be telling you what the product does. Defining the unit by activity makes the same number mean the same thing across products, at the cost of making it untranslatable to weight without a substance-specific factor.
Where you meet IU on a veterinary shelf is mostly predictable: fat-soluble vitamins in injectable and oral preparations, certain hormones, some of the older antibiotics, and heparin. Feed additives and vitamin-mineral premixes frequently state IU per kilogram for vitamins A and D3 alongside milligrams per kilogram for everything else, in the same table — which is not an inconsistency, it is each substance being stated in the unit that is defined for it.
The rule that follows is short. An IU figure can be compared with another IU figure for the same substance, and with nothing else. Converting to milligrams requires the equivalence for that specific substance and form, published by the manufacturer or in a pharmacopoeia monograph. A general conversion table found on the internet is a good way to be precisely wrong.
How do you convert a percentage to mg/mL?
For a weight-in-volume solution, the arithmetic is fixed: 1% means 1 gram of active substance in 100 millilitres of finished product, which is 10 milligrams per millilitre.
The catch is the qualifier, and it is where the mistakes live. A percentage is only a concentration once you know what it is a percentage of:
- w/v — weight in volume. Grams per 100 mL. This is the usual form for injectable and oral solutions, and it is the only one the × 10 rule applies to.
- w/w — weight in weight. Grams per 100 grams. Standard for ointments, creams, powders and in-feed premixes, where there is no volume to divide by.
- v/v — volume in volume. Millilitres per 100 mL, used where the active component is itself a liquid.
A 10% ointment and a 10% injectable solution therefore share a number and nothing else. And an in-feed premix carries a further step: its percentage describes the premix, not the feed, and the concentration an animal actually receives depends on the inclusion rate at which the premix is mixed in.
What are CFU and titre on a biological product?
Live products are measured by counting what survives, because a dead organism has no effect regardless of how much it weighs.
CFU — colony-forming units — counts micro-organisms capable of multiplying into a visible colony under laboratory conditions. It appears on probiotics, live microbial feed additives and live bacterial vaccines, usually as CFU per gram, per millilitre or per dose, and usually in scientific notation: 10⁹ CFU/g is one billion viable organisms per gram.
Titre is not a count but the result of an assay, expressed as the dilution at which a defined effect is still observed. Viral vaccine potency is typically stated this way — TCID50 (the dose infecting 50% of tissue culture units), EID50 (the equivalent in embryonated eggs), or PFU (plaque-forming units) — often written as a log10 value, so a difference of 1 between two figures is a tenfold difference in potency, not a small one.
Both figures describe the product at release and at the end of its shelf life under the stated storage conditions. Neither survives mishandling: potency stated on the carton is a claim about a product that has been kept as the label requires, which is why storage conditions and the cold chain are part of the strength statement in practice even though they are printed elsewhere on the box.
Why concentration matters more than the size of the bottle
The volume of a container tells you how many treatments it holds. It says nothing about how much of the product a single treatment is.
| Presentation | Concentration | Volume | Total active substance |
|---|---|---|---|
| Bottle A | 50 mg/mL | 250 mL | 12 500 mg |
| Bottle B | 200 mg/mL | 100 mL | 20 000 mg |
| Bottle C | 200 mg/mL | 250 mL | 50 000 mg |
Two consequences follow, and both are ordinary rather than exotic. The first is arithmetic: at a fixed amount of active substance per kilogram of body weight, a more concentrated product means a smaller volume administered, which for a large animal can be the difference between one injection site and several. The second is procurement: comparing prices per bottle across two concentrations compares nothing at all, and the comparison that means something is price per unit of active substance.
This is also why switching between two products naming the same substance is not a neutral act — the volume changes even when the substance does not. That comparison is worked through in matching two products by active substance.
Before measuring anything from an unfamiliar container: read the concentration off the label of the container in your hand, not from memory of the last one. Products are reformulated, and the same brand name is sold at more than one strength in many markets — the strength is part of the product's full legal name precisely because it distinguishes two different medicines.
Where the strength is stated in the approved label
Approved veterinary labelling follows a fixed structure in most regulatory systems, and the strength appears in two reliable places.
The first is the product’s full name, which usually includes the strength and the pharmaceutical form: a name of the pattern Substance 100 mg/mL solution for injection for cattle is doing regulatory work, not marketing. The second is the section headed qualitative and quantitative composition, which lists each active substance with its exact amount per unit, and separately lists excipients that need declaring. Where a product contains more than one active substance, each is stated on its own line with its own unit — and it is entirely normal to see milligrams and international units side by side in the same list.
Two further sections carry numbers that are easy to confuse with strength. The amount to be administered section states a dose in terms of active substance per kilogram of body weight; the pharmaceutical particulars section states pack sizes. Neither is the concentration, and a figure copied from the wrong one is a mistake that survives every subsequent step of the calculation.
The checks worth doing before you rely on a number
- Identify which unit the figure is in — mass, percentage, activity or count. If it is a percentage, find whether it is w/v, w/w or v/v.
- Convert only within a unit family. Percentage to mg/mL is safe for w/v; IU to milligrams needs a substance-specific factor; CFU and titre convert to nothing.
- Read the concentration per unit, then work out what the pack contains — not the other way round.
- Check that the strength you are comparing belongs to the same pharmaceutical form. An oral solution and a premix are not comparable on their percentages.
- Check the species and route the strength was authorised for. A concentration is authorised for a use, not in the abstract.
- When two sources disagree, the approved label for the specific product wins. Everything else, including this page, is a description of how the labels are written.
Bottom line
Percentages and milligrams per millilitre are the same statement in two notations, and for a weight-in-volume solution one converts to the other by multiplying by ten. International units are a statement about activity rather than mass, and they convert to milligrams only with a factor specific to that substance. CFU and titre describe living or infective material and convert to nothing at all. Concentration belongs to the medicine and volume belongs to the container, so a bigger bottle is not a stronger product. When a number has to be right, take it from the qualitative and quantitative composition section of the labelling approved for the exact product in front of you, and take the dose it should feed into from the veterinarian responsible for the animal.
Questions readers ask
What does IU mean on a veterinary medicine?
IU stands for international unit. It expresses how much biological activity a quantity of substance has when measured against an internationally agreed reference preparation, rather than how much it weighs. It is used where activity cannot be reliably inferred from mass — vitamins A, D and E, some hormones, some antibiotics and several biological products. Each substance has its own IU-to-milligram equivalence, so a conversion that is correct for vitamin D3 is meaningless for vitamin E.
How do I convert a percentage to mg/mL?
For a weight-in-volume (w/v) solution, 1% means 1 gram in 100 millilitres, which is 10 milligrams per millilitre. So multiply the percentage by 10: a 2.5% solution is 25 mg/mL and a 10% solution is 100 mg/mL. The conversion only holds for w/v. Semi-solid products such as ointments and in-feed premixes are normally expressed weight-in-weight (w/w), where a percentage is grams per 100 grams and no volume is involved.
What is the difference between CFU and titre?
CFU — colony-forming units — counts the micro-organisms in a live product that are capable of growing into a colony, and it is used for probiotics, some feed additives and live bacterial vaccines. A titre is the result of a specific assay, expressed as the dilution at which a defined effect is still observed; viral vaccine potency is often stated as a titre such as TCID50 or EID50 per dose, frequently written as a log10 value. They measure different things by different methods and cannot be converted into one another.
Why does concentration matter more than the size of the bottle?
Because the volume is a property of the packaging and the concentration is a property of the medicine. Two bottles of the same active substance can hold the same total quantity in very different volumes, and the amount to administer depends on the concentration, not on how much liquid the container holds. Reading the bottle size as a measure of strength is one of the more common ways a dose gets miscalculated.
Where on the label is the strength stated?
In the section titled qualitative and quantitative composition, which lists each active substance with the amount present per unit — per millilitre for an injectable or oral solution, per gram for a premix or ointment, per tablet, or per dose for a vaccine. The strength usually also appears in the product's full name. The front of the carton is a summary and is not the place to settle a question about strength.
Does a higher percentage always mean a stronger product?
Only when the two products express the percentage the same way and contain the same active substance. A 10% w/v injectable solution contains 100 mg/mL; a 10% w/w premix contains 100 g per kilogram of premix, which then gets mixed into feed at an inclusion rate that changes the final concentration entirely. Comparing the two numbers directly compares nothing.
Sources
- Veterinary regulatory overview — European Medicines Agency (EMA)
- Animal Drugs @ FDA — approved animal drug products — U.S. Food and Drug Administration
- VICH guidelines on the technical requirements for veterinary medicinal products — VICH (International Cooperation on Harmonisation)
- Critically important antimicrobials for human medicine — World Health Organization
- The Merck Veterinary Manual — MSD/Merck Veterinary Manual
Galen publishes reference material on veterinary medicines. It describes how products are labelled, stored and classified; it does not tell you what to give an animal, in what amount, or for how long. Those decisions belong to a veterinarian who has examined the animal, working from the manufacturer's labelling for the exact product in front of them. Where national rules apply — to prescribing, residues in food, or reporting a suspected reaction — the authority in your own country is the one that governs.