What is the difference between U-40 and U-100 insulin syringes?

The scale. A U-100 syringe is marked so that 100 units fill 1 mL (1 unit = 0.01 mL). A U-40 syringe is marked so that 40 units fill 1 mL (1 unit = 0.025 mL). U-40 syringes are made for U-40 insulin, such as some veterinary insulins; the FDA-approved label for Vetsulin says to use it with U-40 syringes only.

Is there a U-50 syringe?

A "50-unit" syringe is normally a 0.5 mL U-100 syringe: it holds 50 units of 0.01 mL each. It uses the same U-100 scale as 0.3 mL (30-unit) and 1 mL (100-unit) syringes; only the barrel size differs.

Which syringe do compounded GLP-1 unit instructions assume?

U-100. FDA describes a 5-unit dose as 0.05 mL on a U-100 insulin syringe in its July 2024 alert on compounded semaglutide dosing errors. FDA asks prescribers and compounders to provide a syringe sized for the dose and to show patients how to measure it.

How do you read half-unit marks?

Some 0.3 mL U-100 syringes have a half-unit scale, with a line every 0.5 unit (0.005 mL). Count lines from zero to the top edge of the plunger seal. On a 1 mL U-100 syringe the lines are usually every 2 units, so a dose like 5 units falls between lines and is harder to measure.

U-40 vs U-100 Insulin Syringes: The 2.5x Dosing Error

Q: What happens if you use a U-40 syringe for a U-100 dose?

Drawing to the same number gives 2.5 times the volume (0.025 mL ÷ 0.01 mL per unit), so 2.5 times the intended amount of drug. A published case report describes a patient who drew U-100 insulin into U-40 veterinary syringes and had hypoglycemia for this reason. Going the other way, a U-100 syringe used for a U-40 product gives 0.4 times the intended amount.

Short answer: On a U-100 syringe, 1 unit is 0.01 mL and 100 units fill 1 mL. On a U-40 syringe, 1 unit is 0.025 mL and 40 units fill 1 mL. So "10 units" on a U-40 syringe is 2.5 times as much liquid as "10 units" on a U-100 syringe. Compounded GLP-1 unit instructions assume U-100.

How do U-100 and U-40 syringe scales compare?

Same number on the barrel, different volume.
Marked units	U-100 volume	U-40 volume	U-40 ÷ U-100
2 units	0.02 mL	0.05 mL	2.5x
5 units	0.05 mL	0.125 mL	2.5x
10 units	0.1 mL	0.25 mL	2.5x
20 units	0.2 mL	0.5 mL	2.5x
25 units	0.25 mL	0.625 mL	2.5x
50 units	0.5 mL	1.25 mL	2.5x

U-40 syringes exist for U-40 insulin, including veterinary insulin; the FDA-approved Vetsulin owner sheet says to use it with U-40 syringes only. They look much like U-100 syringes, which is how mix-ups happen. A 2019 case report describes a patient who drew U-100 insulin into U-40 veterinary syringes and got 2.5 times the intended dose.

What do 0.3 mL, 0.5 mL and 1 mL syringes hold?

U-100 insulin syringes. All use the same scale; only the barrel size differs.
Barrel	Holds	1 unit	Notes
0.3 mL	30 units	0.01 mL	Half-unit scale available on some brands
0.5 mL	50 units	0.01 mL	Often called a "50-unit" syringe
1 mL	100 units	0.01 mL	Lines usually every 2 units, numbered every 10

A "50-unit syringe" is not a different scale. It is a 0.5 mL U-100 syringe. Smaller barrels spread the same volume over a longer stretch of barrel, which makes small doses easier to read. FDA's 2024 alert asks prescribers and compounders to give patients a syringe sized for the intended dose.

How do you read the tick marks?

Check the scale on the barrel or the box says U-100. If it says U-40, the unit instructions for a compounded GLP-1 do not apply.
Find the number on your instructions, then count lines from zero. On a 1 mL syringe, each small line is usually 2 units; on a 0.3 mL half-unit syringe, each small line is 0.5 unit.
Read at the top edge of the black plunger seal (the edge nearest the needle), not the bottom of it.
Tap out large air bubbles and recheck; a bubble takes up space that should be liquid.

If your dose works out to a fraction that is not on a line (for example 31.25 units), it cannot be measured exactly on that syringe. Ask the prescriber or pharmacy how they want it rounded rather than guessing.

What did FDA say about compounded GLP-1 dosing errors?

In July 2024 FDA alerted health care providers, compounders and patients to dosing errors with compounded injectable semaglutide. FDA had received adverse event reports, some needing hospitalization, linked to overdoses. Its examples:

Patients drew more than prescribed from multi-dose vials, in some cases 5 to 20 times the intended dose. One drew 50 units on a U-100 syringe instead of 5 units (0.05 mL).
A provider meant 0.25 mg (5 units) but prescribed 25 units, a 5-fold overdose.
A provider prescribed 20 units instead of 2 units, a 10-fold overdose.

FDA linked the errors to unfamiliarity with drawing from a vial and confusion between milligrams, milliliters and units, and noted that compounded semaglutide comes in different concentrations. FDA's page on unapproved GLP-1 drugs also notes adverse events reported with compounded semaglutide and tirzepatide, and that compounded drugs are not FDA approved.

How does this apply to semaglutide and tirzepatide vials?

Units are volume. The amount of drug in each unit depends on the vial strength, so always convert with the strength on the label: mg ÷ mg/mL × 100 = U-100 units. Charts for every label dose step: semaglutide units by strength and tirzepatide units by strength. FDA-approved Ozempic, Wegovy, Mounjaro and Zepbound pens are dosed in mg and need no unit math. For vials you mix yourself, see the reconstitution calculator and insulin syringe units explained.

Not personalized advice; confirm your dose, in mg and in units, with your prescriber or the dispensing pharmacy.

U-40 vs U-100 insulin syringes: what is the difference?