U-40 vs U-100 syringes: what’s the difference?

A U-40 syringe is scaled for insulin that holds 40 units per mL, so each unit on its scale is 0.025 mL; a U-100 syringe is scaled for 100 units per mL, so each unit is 0.01 mL (Case Rep Endocrinol, 2019). The numbers on the barrel look alike, but the volumes behind them differ by 2.5 times, so a syringe only reads correctly with the liquid it was made for.

In the US, U-40 insulin is made for animals, and U-100 is the standard strength for people (Case Rep Endocrinol, 2019). Every Peptinote calculator, including the one that shows how to read an insulin syringe, assumes a U-100 syringe. If the barrel or box says U-40, none of that math applies.

What the U-number means

Insulin strength is written as units of insulin per mL. Human insulin labeled “U-100” has 100 units in every mL of liquid, and it’s the standard strength in the US (Case Rep Endocrinol, 2019). The Humulin R U-100 carton reads “100 units per mL (U-100)” and “Use only with a U-100 syringe” (Humulin R label).

An insulin syringe’s scale is printed to match one strength. On a U-100 syringe, the units are a plain volume scale: 100 units = 1 mL. On a U-40 syringe, the same word “units” means something else, because 40 of its units fill 1 mL. Vetsulin’s owner sheet puts it this way: “U-40 insulin contains 40 units/mL (1 mL = 1 cc)” (Vetsulin label).

The small printed lines aren’t always one unit apart. Clinicians in India describe U-100 syringes with lines “each denoting 2 units” and U-40 syringes with lines each denoting 1 unit (Chowdhury and Chakraborty, 2017). Some syringes add half-unit lines. Count by the printed numbers, and see what each tick mark means on a U-100 syringe.

U-40 vs U-100 at a glance

Sources: Case Rep Endocrinol, 2019, the Vetsulin, ProZinc and Humulin R labels.

Where U-40 syringes come from

U-40 insulin doesn’t exist for humans in the US, but it’s a very common strength for animals (Case Rep Endocrinol, 2019). Two FDA-approved veterinary insulins are U-40, and both labels carry the same warning in capitals:

• Vetsulin (porcine insulin zinc suspension, for dogs and cats) is “40 units per mL (U-40)”. Its label says: “USE OF A SYRINGE OTHER THAN A U-40 SYRINGE WILL RESULT IN INCORRECT DOSING” (Vetsulin label).
• ProZinc (protamine zinc recombinant human insulin, for cats and dogs) contains 40 IU per mL and carries the same sentence (ProZinc label).

So U-40 syringes turn up wherever pet insulin is handled: a veterinary clinic, a pet pharmacy, a household with a diabetic dog or cat. Outside the US, some countries have sold only U-40 insulin (Case Rep Endocrinol, 2019). A 2017 report of clinical experiences from India describes U-40 and U-100 vials and syringes both in use there (Chowdhury and Chakraborty, 2017).

What a mix-up does

Read one kind of syringe as if it were the other, and every reading is 2.5 times off. Both directions have been reported:

• U-100 liquid in a U-40 syringe: too much. Drawing U-100 insulin to a mark on a U-40 syringe gives 2.5 times the intended amount. A man treated in Illinois who used veterinary U-40 syringes with his prescribed U-100 insulin had repeated low blood sugars and erratic control (Case Rep Endocrinol, 2019).
• U-40 liquid in a U-100 syringe: too little. A patient in India drew from a U-40 vial with a U-100 syringe and got 2.5 times less insulin than prescribed; his blood sugars ran high until the mismatch was found (Chowdhury and Chakraborty, 2017).

The table shows the volumes involved, using an example strength of 5 mg/mL (numbers for illustration only, not a dose). A U-40 syringe filled to its 40 mark holds 1 mL, the same volume as the 100 mark on a U-100 syringe. Someone reading that U-40 syringe with U-100 math would expect the 40 row below; they would actually get the 100 row. In the same way, the 10 mark on a U-40 syringe holds the volume of the 25 row.

The amount in each syringe reading at 5 mg/mL.

Reading	Volume	Amount
10 units	0.1 mL	500 mcg
25 units	0.25 mL	1.25 mg
40 units	0.4 mL	2 mg
100 units	1 mL	5 mg

U-100 syringe: 100 units = 1 mL. At a different strength the same reading holds a different amount.

Why it matters for peptide and GLP-1 vials

When a pharmacy or prescriber writes an amount in “units” for a vial, the number only means something on the syringe it was written for. In the FDA’s 2024 alert on compounded semaglutide, most reports described patients drawing up five to 20 times the intended dose, and the FDA said confusion between mL, mg and “units” may have contributed. In several reports, people told to draw 5 units on a U-100 syringe drew 50 instead (FDA).

A U-40 syringe adds another way to get it wrong, because “20 units” on it is not the same volume as “20 units” on a U-100 syringe. Peptinote’s units converter and every other calculator here use U-100 math only, as explained in how our calculator works. If your instructions are in units, confirm with your pharmacist which syringe they assume.

How to tell them apart

1. Read the printed U-number

   Look on the barrel and the box for “U-40” or “U-100”. This is the only reliable check.

2. Match it to the label

   Insulin labels name the syringe: “Use only with a U-100 syringe” on Humulin R, “U-40 syringes only” on Vetsulin and ProZinc. Peptide math on this site is for U-100.

3. Don’t trust color alone

   The US case report says U-40 syringes “most of the time” have a red cover and red scale, unlike the orange cover and black scale of U-100 (Case Rep Endocrinol, 2019). Authors in India add that the red and orange caps can be hard to tell apart (Chowdhury and Chakraborty, 2017). Treat color as a hint, never as the check.

4. Ask if you’re unsure

   If the U-number is missing or rubbed off, don’t use the syringe. Ask your pharmacist.

There’s also a third scale. Concentrated U-500 insulin has its own syringe with “a green U-500 symbol and a green Needle Shield”, and its label warns that using a U-100 syringe instead “can give you a 5 times overdose” (Humulin R U-500 label).

Common mistakes

• Using pet-care syringes for a human or peptide vial. Most pet owners use U-40 syringes (Case Rep Endocrinol, 2019), and the math won’t match.
• Using a U-100 syringe for a pet’s U-40 insulin. The Vetsulin and ProZinc labels say this gives incorrect dosing.
• Going by cap color. Red and orange look alike; read the U-number.
• Taking a “units” instruction without asking which syringe it’s for.
• Picking up syringes abroad without checking the scale. The 2019 case report says travelers from North America must bring their own insulin and syringes, since some countries sell only U-40 insulin.

Questions people ask