U-40 to U-100 Insulin Syringe Converter for Dogs and Cats

Insulin syringe converter

Insulin your vet prescribed

Dose prescribed by your vet (IU)

Syringe you have

Syringe size

Draw up to this mark on your syringe:4marks

Volume0.10 mL

Insulin strengthU-40

Syringe and insulin

Educational tool only. It converts a dose that a veterinarian has already prescribed. It does not choose or change a dose. Always use the syringe or pen your vet recommends for your insulin.

How to use this converter

1

Choose the insulin

Read the strength on the vial or the box: U-40 means 40 IU per mL, U-100 means 100 IU per mL.

2

Enter the prescribed dose

Type the dose in international units (IU), exactly as your vet wrote it. The tool never suggests a dose.

3

Choose your syringe

The syringe type is printed on the barrel or the packet: U-40 or U-100.

4

Read the result

You see the mark to draw to, the volume in mL and a warning if the syringe and the insulin do not match.

Insulins used in dogs and cats and their syringes

Veterinary insulins are U-40. Most human insulins that vets prescribe for cats and dogs are U-100. The strength of the insulin and the scale of the syringe must be the same.

Insulin	Type	Strength	Matching syringe
Vetsulin (US) / Caninsulin (EU, UK, Australia)	Porcine lente insulin, veterinary, dogs and cats	40 IU/mL	U-40 or VetPen
ProZinc	Protamine zinc insulin (PZI), veterinary, cats and dogs	40 IU/mL	U-40
Lantus (glargine)	Human insulin, prescribed mostly for cats	100 IU/mL	U-100
Toujeo (glargine)	Human insulin	300 IU/mL	Pen only, not interchangeable with Lantus
Levemir (detemir)	Human insulin, discontinued in the US after 31 December 2024	100 IU/mL	U-100
Humulin N / Novolin N (NPH)	Human insulin	100 IU/mL	U-100

Merck Animal Health states that U-100 syringes deliver less than half of the required Vetsulin dose. The VetPen for Vetsulin and Caninsulin exists in two versions: 8 IU (steps of 0.5 IU) and 16 IU (steps of 1 IU).

Why a syringe mismatch is dangerous

A syringe mark is a volume, not a quantity of insulin. On a U-100 syringe, one mark is 0.01 mL. On a U-40 syringe, one mark is 0.025 mL. The marks show insulin units only when the syringe and the insulin have the same strength.

U-40 insulin in a U-100 syringe: if you draw to the prescribed number, your pet receives only 40 % of the dose. Example: 4 marks of Vetsulin on a U-100 syringe give 1.6 IU instead of 4 IU. The diabetes stays uncontrolled.
U-100 insulin in a U-40 syringe: your pet receives 2.5 times the dose. Example: 4 marks of Lantus on a U-40 syringe give 10 IU instead of 4 IU. This can cause severe hypoglycemia.

Signs of hypoglycemia include weakness, trembling, a wobbly walk, confusion and seizures. If you see them, contact your vet or an emergency clinic immediately.

AAHA guidelines give a simple rule: always use a U-40 syringe with U-40 insulin and a U-100 syringe with U-100 insulin. Use the converter only to check, or when your vet tells you to use a specific syringe.

The formula

Volume to draw:

Volume (mL) = Dose (IU) ÷ Insulin strength (IU/mL)

Mark on the syringe:

Marks = Dose (IU) × Syringe scale ÷ Insulin strength

With a matched pair, the factor is 1 and the mark equals the dose. U-40 insulin in a U-100 syringe gives a factor of 2.5. U-100 insulin in a U-40 syringe gives a factor of 0.4.

Good practice for insulin injections at home

Never change the insulin brand, the strength or the syringe type without your vet.
Check the strength on each new vial, and the type on each new box of syringes.
Ask the clinic for a demonstration of the first injections.
Write down each injection (time, dose, appetite) and bring the log to each visit.
Weight affects diabetes control, especially in cats. To estimate daily calories, use our dog and cat calorie calculator.

Sources

Behrend E, Holford A, Lathan P, Rucinsky R, Schulman R. 2018 AAHA Diabetes Management Guidelines for Dogs and Cats. J Am Anim Hosp Assoc 54(1):1-21, 2018.
Bugbee A et al.. 2026 AAHA Diabetes Management Guidelines for Cats. J Am Anim Hosp Assoc 62(3):65-93, 2026.
Taylor S et al.. iCatCare 2025 consensus guidelines on the diagnosis and management of diabetes mellitus in cats. J Feline Med Surg 27(11), 2025.
Merck Animal Health. Vetsulin: syringe administration and VetPen. Manufacturer information, 2026.
MSD Animal Health UK. Caninsulin 40 IU/ml suspension for injection. Product page, 2026.
Boehringer Ingelheim. ProZinc (protamine zinc recombinant human insulin) label. Drugs.com Vet, 2026.
Eli Lilly. Humulin N label. DailyMed, 2026.
Sanofi. Toujeo (insulin glargine 300 units/mL) prescribing information. FDA label, 2018.

Frequently Asked Questions

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

U-40 insulin contains 40 international units (IU) per mL. U-100 insulin contains 100 IU per mL. The veterinary insulins Vetsulin, Caninsulin and ProZinc are U-40. Human insulins such as Lantus, Levemir and NPH are U-100.

Can I use a U-100 syringe with Vetsulin or Caninsulin?

Only if your vet tells you to and shows you the conversion. If you read the marks as units, a U-100 syringe gives less than half the dose. To deliver a dose of U-40 insulin with a U-100 syringe, the mark is the dose multiplied by 2.5.

What happens if I use a U-40 syringe with Lantus?

If you draw to the prescribed number, your cat or dog receives 2.5 times the dose, which can cause severe hypoglycemia. Lantus must be drawn with a U-100 syringe.

Which syringe goes with ProZinc?

ProZinc is a U-40 insulin (40 IU/mL), so its label specifies U-40 syringes.

Can I use Toujeo in place of Lantus for my cat?

No, not without your vet. Toujeo is U-300 glargine and is not interchangeable with U-100 Lantus. The 2025 iCatCare guidelines state that U-300 can only be given with a dosing pen.

Does this tool tell me how much insulin to give?

No. Only your vet can set an insulin dose, from blood glucose curves, clinical signs and weight. The tool converts the prescribed dose into a mark on the syringe you have.

The result falls between two marks. What do I do?

Do not guess. Ask your vet how to measure that volume. A half-unit syringe or a dosing pen can be a solution.

U-40 / U-100 Insulin Syringe Converter for Dogs and Cats