What the numbers mean
The U number is the insulin's strength in units per mL. Standard human insulin is U-100, 100 units in each mL, and most insulin syringes are marked for it.[3] Veterinary insulins for dogs and cats, such as Vetsulin and ProZinc, are U-40: Vetsulin's label reads “40 units per mL (U-40)”, and each mL of ProZinc contains 40 units.[1][2] Insulin syringe scales are made for one of the two strengths, U-100 or U-40.[6]
| U-40 | U-100 | |
|---|---|---|
| Units per mL | 40 | 100 |
| Volume of 1 unit | 0.025 mL | 0.01 mL |
| 1 mL syringe holds | 40 units | 100 units |
| 0.5 mL syringe holds | 20 units | 50 units |
| 0.3 mL syringe holds | 12 units | 30 units |
| Needle shield colour (ISO 8537, embecta) | Red | Orange |
| Typical use in the US | Veterinary insulin | Human insulin |
U-100 syringe (1 mL) at the 10 line · 0.1 mL
U-40 syringe (1 mL) at the 10 line · 0.25 mL
The labels: U-40 insulin, U-40 syringe
Both US veterinary insulin labels carry the same capitalised warning: use of a syringe other than a U-40 syringe will result in incorrect dosing. Vetsulin's label says to use it with U-40 syringes only, and ProZinc's says to give it with U-40 syringes only.[1][2]
Never swap the scalesUse U-40 syringes with U-40 insulin and U-100 syringes with U-100 insulin, as the labels direct. If you have the wrong syringes, ask your vet or pharmacist before giving a dose.
What a mix-up does
- U-40 insulin in a U-100 syringe: too little. Drawing to the 10 line takes 0.1 mL, which holds 4 units of U-40 insulin: 40% of the dose. Merck Animal Health says a U-100 syringe with Vetsulin gives the animal 2½ times less insulin than required.[4][5]
- U-100 insulin in a U-40 syringe: too much. Drawing to the 10 line takes 0.25 mL, which holds 25 units of U-100 insulin: 2.5 times the dose. Merck warns that this could result in fatal hypoglycemia.[4] A published case report describes a person who used U-40 veterinary syringes with U-100 insulin and received 2.5 times the intended dose.[8]
units given = line drawn to × (insulin strength ÷ syringe scale)
U-40 insulin, U-100 syringe: × 0.4 · U-100 insulin, U-40 syringe: × 2.5
The U-40 conversion chart and checker works this out for any number of units. For the volume of any number of units on each scale, see 1 mL = how many units.
Telling them apart
- Print. The scale, U-40 or U-100, is printed on the barrel and the box. This is the check that counts.
- Colour. embecta follows ISO 8537 with an orange needle shield for U-100 and red for U-40.[6] BD's U-40 pet syringes also have red scale marks.[7] Colour is a helpful cue, not a guarantee.
- Sizes. BD's U-40 pet syringes come in 0.3 mL and 0.5 mL; embecta also lists 0.5 mL and 1 mL U-40 syringes.[7][6]
More on barrel sizes and markings is in insulin syringe sizes, and other scale and unit mix-ups are collected in syringe measuring mistakes.
The American Diabetes Association notes that U-40 markings still exist in a few parts of the world, for older insulin concentrations and for veterinary insulin.[9]
Frequently asked questions
What is the difference between U-40 and U-100 syringes?
A U-40 syringe is marked for insulin with 40 units per mL, so each unit line is 0.025 mL. A U-100 syringe is marked for 100 units per mL, so each line is 0.01 mL.
Can I use a U-100 syringe for my dog or cat's U-40 insulin?
The Vetsulin and ProZinc labels say to use U-40 syringes only. A U-100 syringe would give 2.5 times less insulin if you drew to the same number. Ask your vet.
What colour are U-40 syringes?
Under ISO 8537, embecta uses a red needle shield for U-40 and orange for U-100. Always confirm the printed scale.
Sources
- Vetsulin (porcine insulin zinc suspension) label
- ProZinc (protamine zinc recombinant human insulin) label
- Giving an insulin injection
- Vetsulin frequently asked questions
- Vetsulin syringe administration (cats)
- Technical data sheet: insulin syringes (Micro-Fine, Ultra-Fine)
- BD extends longstanding expertise in diabetes management to benefit pets
- Barrera F, Murvelashvili N. Poorly controlled type 3c diabetes due to use of veterinarian insulin syringes
- 7. Diabetes technology: Standards of Medical Care in Diabetes—2022