What Is a U-40 Syringe? The 2.5x Mix-Up Explained
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[Glossary · Mixing and measuring]

U-40 syringe

Also called U-40 insulin syringe, U40 syringe, pet insulin syringe, veterinary insulin syringe · Reviewed

Definition: A U-40 syringe is an insulin syringe marked for insulin that holds 40 units per mL. Each unit mark equals 0.025 mL, so 40 marks fill 1 mL. In the US it is used mainly with veterinary insulins for dogs and cats. Its marks are 2.5 times larger than a U-100 syringe's.

What the label says

Vetsulin, a pet insulin, is 40 International Units per mL. Its label warns in capitals: "use of a syringe other than a U-40 syringe will result in incorrect dosing." Human insulin such as Humulin R is 100 units/mL and is labeled for U-100 syringes only. The syringe has to match the insulin.

Worked example: what a mismatch does

U-100 liquid drawn on a U-40 syringe

  1. Draw to the 10 mark on a U-40 syringe: 10 × 0.025 mL = 0.25 mL.
  2. With U-100 insulin, 0.25 mL holds 0.25 × 100 = 25 units.
  3. The intended amount was 10 units. The result is 2.5 times too much.

U-40 insulin drawn on a U-100 syringe

  1. Draw to the 10 mark on a U-100 syringe: 10 × 0.01 mL = 0.1 mL.
  2. With U-40 insulin, 0.1 mL holds 0.1 × 40 = 4 units.
  3. The intended amount was 10 units. The result is 40% of it.

The same 2.5-times error applies to peptide math. If you worked out "10 units" for a U-100 syringe and measure it on a U-40 syringe, you draw 0.25 mL instead of 0.1 mL.

Common mix-ups

A 2019 case report described a man who drew his U-100 insulin into U-40 veterinary syringes and had repeated low blood sugar episodes. The authors note that U-40 strength is common in veterinary use but not sold for humans in the US.

  • Buying syringes from a pet supply source by accident.
  • Assuming all insulin syringes are the same because they look alike. Check for "U-40" or "U-100" on the package and barrel.

Read more on Dose Track: U-40 vs U-100 insulin syringes · Peptide reconstitution calculator

General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
U-100 syringeA U-100 syringe is an insulin syringe marked for insulin that holds 100 units per mL. Each unit mark… Insulin syringe unitsThe units on an insulin syringe are marks of volume. On a U-100 syringe, 1 unit = 0.01 mL; on a U-40… Concentration (mg/mL)Concentration is how much drug is in each unit of volume, usually written as mg/mL, mcg/mL or units/mL. It… International unit (IU)An international unit (IU) measures a drug by its biological activity, compared with an agreed reference…

Frequently asked questions

How many mL is 1 unit on a U-40 syringe?+
One unit mark is 0.025 mL. 40 marks fill 1 mL.
What happens if I use a U-40 syringe with U-100 insulin?+
Each mark holds 2.5 times more U-100 insulin than intended. Drawing to 10 gives 25 units.
Who uses U-40 syringes?+
In the US they are used mainly with veterinary insulins such as Vetsulin, which is 40 IU per mL.

Sources

  1. Vetsulin (porcine insulin zinc suspension) 40 IU/mL label, DailyMed
  2. Humulin R U-100 (insulin human) US prescribing information, DailyMed
  3. Barrera and Murvelashvili. Case Rep Endocrinol 2019 (U-100 insulin drawn into U-40 veterinary syringes). PMID 31316841

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