Research
What a strength on a vial actually states
The number printed on an injectable can answer either of two questions: how much drug is in the container, or how much is in each milliliter of it. Both are called the strength, only one of them is the denominator a price comparison needs, and FDA has published that compounders and manufacturers do not always choose the same one.
One word, two different questions
A strength on an injectable label is a quantity of drug set against a quantity of liquid. There are two ordinary ways to write it, and they are not the same statement.
One states the total drug in the container. The other states the drug in each milliliter, which is a concentration. A container can carry both, and many do.
Neither is wrong and neither is unusual. The difficulty is that both get called the strength, and printed on their own they look alike. A figure with no unit beside it does not tell you which question it answered.
Everything in this article follows from that. To turn a price into a price per milligram, you need the total in the container. A concentration alone does not give you one without the volume.
What FDA recommends, and who it recommends it to
The agency has stated a preference between the two, and the wording is worth reading closely.
For small-volume injection products, FDA recommends that the product strength be expressed as quantity per total volume. That expression should be the primary and prominent one on the principal display panel of the label. Quantity per milliliter should follow it, in parentheses.
The recommendation sits in a final guidance issued in May 2022 on container labels and carton labeling design. That guidance describes its own audience: human prescription drug and biological product sponsors, application holders and applicants.
In other words, it is addressed to the companies that hold approved applications. It is not a rule imposed on compounding pharmacies, and reading it as one would be a mistake.
What the agency has published about compounded labels
FDA has also published a separate alert about this exact difference, and its subject is patient safety rather than shopping.
The agency states there that conventional manufacturers label their injectable products with the strength per total volume as the primary and prominent expression, whereas some compounders label their injectable products differently. The alert exists because the agency received reports of dosing errors and confusion tied to how strength was expressed.
The errors described in it happened in clinical settings, where a health care professional read a prominent per-milliliter figure as the total in the container. This site does not reproduce those case reports, and nothing here is a safety claim about anything on the roster.
The alert also records two plain facts about the category. Compounded drugs are not evaluated by FDA for safety, effectiveness and quality, and labels of compounded products are not reviewed by FDA before marketing.
What it does not do is require compounders to change. It says health care professionals may consider requesting that compounders use the same convention as manufacturers.
Why a comparison shopper ends up in the same place
The clinical hazard and the pricing problem come from one source, which is that the prominent number may not be the total.
Price per milligram is the unit that lets two sellers of one compound be set against each other, and it is built by dividing a price by the milligrams in the container. That division needs the total content.
When a listing gives a concentration and no volume, there is no total and the unit cannot be built. When a listing gives a concentration and a volume, the total is recoverable, but you have to notice that you are doing it.
And when a listing gives a bare figure with no unit at all, it is not a strength. It is a number that looks like one.
Two listings can print the same figure and hold different amounts
This is the failure that matters, and it needs no arithmetic to see.
One seller prints a total content. Another prints a concentration. Both figures happen to read the same, so a reader scanning two tabs concludes the vials are equivalent.
They are equivalent only if the second container holds exactly one milliliter. Otherwise the two listings describe different amounts of drug, and the cheaper of the two is cheaper for a reason that has nothing to do with the seller being better value.
The check is not difficult. Read the unit, not the digits. If the figure carries a per-milliliter unit, find the volume before you compare anything.
The strength is settled after the intake, and the price may move with it
A compounded preparation is made against a prescription, so the strength is not a shelf choice. It is decided as part of the clinical process and it can change later.
That has a direct effect on what a published figure is worth to you. A price shown beside one strength describes a purchase you have not been prescribed yet.
Sellers handle this in two visibly different ways, and the difference is a real comparison axis. Some publish one rate for a compound at every strength, so a change to the prescription changes what is supplied rather than what is billed. Others publish a separate rate for each strength, so the figure moves when the prescription does.
Neither structure is better in the abstract, and the second is arguably more honest about the underlying cost. But they cannot be compared on a single advertised figure, because one of the two figures is conditional on a decision that has not been made.
The practical question to ask before paying is what the rate would be at the strengths a seller offers, and whether a change is billed as a change.
What to check before setting two listings side by side
Read the unit attached to every figure, and treat a figure without a unit as unusable rather than as a total.
Where a concentration is given, look for the container volume on the same page. Without it there is no total and no comparable unit.
Check that both listings are describing the same delivery format before anything else, because a comparison across formats does not become valid by fixing the strength.
Ask whether the published rate holds at every strength the seller offers, or only at the one shown.
Ask what happens to the price if the prescription changes, and get the answer before the first charge rather than after the second.
Your own strength and your own instructions come from your prescriber and from the labeling supplied with your medication. Nothing here describes what anyone should take.
Key takeaways
- A strength on an injectable states either the total drug in the container or the drug in each milliliter, and both are called the strength.
- Only the total works as the denominator in a price per milligram, so a concentration alone does not produce a comparable figure.
- FDA recommends quantity per total volume as the primary and prominent expression on small-volume injection labels, with quantity per milliliter in parentheses.
- That recommendation is addressed to holders of approved applications, not to compounders, and reading it as a requirement on compounders is wrong.
- FDA has published that some compounders label injectables differently from conventional manufacturers, in an alert about dosing errors rather than about price.
- Two listings printing the same digits can hold different amounts of drug when one figure is a concentration and the other is a total.
- The strength on a compounded preparation is settled after the intake, so an advertised figure describes a purchase that has not been prescribed yet.
Frequently asked questions
What is the difference between a strength written per milliliter and one written as a total?
They answer different questions about the same container. A total states how much drug is inside it. A per-milliliter figure states a concentration, so the total depends on how many milliliters the container holds. Both are commonly called the strength. Only the total works as the denominator in a price per milligram, which is the unit that lets two sellers of the same compound be compared.
Which figure does FDA recommend be shown most prominently?
For small-volume injection products, FDA recommends that strength be expressed as quantity per total volume, and that this be the primary and prominent expression on the principal display panel. Quantity per milliliter should follow it in parentheses. That recommendation appears in a final guidance issued in May 2022 whose own stated audience is human prescription drug and biological product sponsors, application holders and applicants.
Does that recommendation apply to compounded vials?
Not as a requirement. The guidance is addressed to holders of approved applications. In a separate alert, FDA states that conventional manufacturers use strength per total volume as the primary and prominent expression, whereas some compounders label their injectable products differently. The same alert says health care professionals may consider requesting compounders to use the manufacturers' convention. The same alert notes that labels of compounded products are not reviewed by FDA before marketing.
Can I work out a price per milligram from a concentration alone?
No. A concentration tells you how much drug is in each milliliter and says nothing about how many milliliters you are buying. Without the container volume there is no total, and without a total there is no price per milligram. A listing that publishes a concentration and no volume has not published a comparable figure, however precise the number looks.
Two sellers list the same strength. Am I getting the same amount of drug?
Only if both figures are the same kind of figure. If one is a total content and the other is a per-milliliter concentration, the two containers hold the same amount only when the second holds exactly one milliliter. Read the unit rather than the digits, and find the volume whenever a concentration is what is printed.
Why might my price change after the consultation?
Because a compounded preparation is made to a prescription, and the strength is settled as part of that process rather than picked from a shelf. Some sellers publish one rate for a compound at every strength, so a change to the prescription changes what is supplied rather than what is billed. Others publish a separate rate per strength. Ask which arrangement applies, and ask what a later change would be billed as.
Sources
Each document below is named as it names itself, with the date printed on that document rather than the day it was read.
- Differences in strength expression on product labels of compounders and conventional manufacturers may lead to dosing errors — compounding risk alert, including the statement that conventional manufacturers use strength per total volume as the primary and prominent expression whereas some compounders label their injectable products differently — U.S. Food and Drug Administration, June 2024
- Safety Considerations for Container Labels and Carton Labeling Design to Minimize Medication Errors — final guidance, May 2022, whose stated audience is human prescription drug and biological product sponsors, application holders and applicants — U.S. Food and Drug Administration, Center for Drug Evaluation and Research, May 2022