Units versus mL: the mistake that changes a dose tenfold

Three different things get called a "unit" around a vial of compounded medication, and the confusion between them is the most common way people get a dose wrong.

The three meanings

1. A syringe unit. The graduations printed on an insulin syringe. On a U-100 syringe, 100 of these marks span 1 mL, so each mark is 0.01 mL. This is a measure of volume. The syringe has no idea what is inside it.

2. An insulin unit. A measure of insulin activity. U-100 insulin is manufactured at 100 insulin units per mL, which is why the syringes were designed the way they are: 1 mark, 1 insulin unit. That correspondence is a property of insulin, not of the syringe.

3. "Units" on a compounded GLP-1 label. When a pharmacy prints "draw 25 units" on a semaglutide or tirzepatide vial, it means syringe marks, meaning 1: a volume of 0.25 mL. It is a translation of the milligram dose for that vial's concentration. There is no such thing as a unit of semaglutide in the way there is a unit of insulin.

Where the tenfold errors come from

The mistake is rarely a wrong calculation. It is applying a right number to the wrong scale.

• Reading mL on the units scale. A prescription written as 0.25 mL, drawn to the 25 mark, is correct. Drawn to the 2.5 mark it is a tenth of the dose. Drawn to the "0.25" printed on a tuberculin syringe's mL scale it is correct again. Same number, three different physical amounts.
• Carrying units across a concentration change. 25 units of a 10 mg/mL vial is 2.5 mg. 25 units of a 5 mg/mL vial is 1.25 mg. A refill at a different concentration silently changes what the old instruction delivers.
• Confusing mg with mL. "2.5" on a prescription means 2.5 mg. Drawn as 2.5 mL from a 10 mg/mL vial, it would be 25 mg, ten times the dose. This is the pattern the FDA describes in its adverse event summary for compounded semaglutide.

The arithmetic that prevents it

Do the conversion in this order every time, and write it down.

1. Concentration, from the vial label: C mg/mL.
2. Dose, from the prescription: D mg.
3. Volume: V = D / C mL.
4. Syringe marks: V x 100 units (U-100 syringe only).
5. Sanity check: is V below 1 mL, and is D the number on the prescription? If either answer is no, stop.

For 2.5 mg from 10 mg/mL: V = 0.25 mL, 25 units. For 0.5 mg from 2.5 mg/mL: V = 0.2 mL, 20 units. For 5 mg from 10 mg/mL: V = 0.5 mL, 50 units.

The dose unit converter does exactly these steps and shows them. Use it as a check on your own arithmetic, not a replacement for it.

A note on syringe types

Not every small syringe is a U-100 insulin syringe. A 1 mL tuberculin syringe is graduated in hundredths of a millilitre and has no units scale at all. A U-40 insulin syringe, sold for veterinary insulin, has 40 marks per mL, so each mark is 0.025 mL, and using one with U-100 instructions gives 2.5 times the intended volume. If your syringe says anything other than U-100, do not use a units figure computed for U-100.

If you think you have made an error

Do not take a second injection to "correct" a suspected underdose, and do not wait to see what happens after a suspected overdose. Call your pharmacist or prescriber, or the poison control line in your country, and tell them the concentration, the volume you drew, and when. Compounded semaglutide and tirzepatide are not FDA approved, and the reported errors are one of the reasons the FDA has raised concerns about them; that is context for taking the arithmetic seriously, not a reason to guess.