U-100 vs U-40 syringes: why the scale matters
U-100 and U-40 describe how concentrated the insulin is, and the syringe scale is printed to match. Human insulin in Australia is U-100. U-40 is a veterinary strength. A syringe from the wrong family reads out a dose 2.5 times too high or too low.
What the numbers mean
U-100 means 100 units of insulin in each millilitre. U-40 means 40 units per millilitre. A U-100 syringe is marked so that 1 ml reads 100 units; a U-40 syringe is marked so that 1 ml reads 40 units. The barrels hold the same liquid volume but the printed scales disagree by a factor of 2.5.
What goes wrong
| You draw up | Using | Actual volume | Actual dose if the insulin is U-100 |
|---|---|---|---|
| 20 units | U-100 syringe | 0.20 ml | 20 units |
| 20 units | U-40 syringe | 0.50 ml | 50 units |
Same intended number, two and a half times the medicine. The reverse mistake, U-40 insulin in a U-100 syringe, gives 40 percent of the intended dose. Both are serious. This is the reason the two syringe types are colour coded: U-100 caps are orange, U-40 caps are red.
Who this affects in practice
In Australia, human insulin is U-100, and the syringes sold for people are U-100. U-40 turns up in veterinary use for cats and dogs, whose insulin is often that strength. The risk is a household with both: a person and a pet on insulin, syringes in the same drawer. If that is your house, keep the two in separate, labelled places.
How to tell them apart
- The printing. The barrel says U-100 or U-40, usually near the scale. Check every new box.
- The cap colour. Orange for U-100, red for U-40, across the major brands.
- The top number on the scale. A 1 ml U-100 syringe reads to 100. A 1 ml U-40 syringe reads to 40.
Every tool on this site, including the converter, assumes U-100. Nothing here applies to a U-40 syringe.
Common questions
- Can I use a U-100 syringe for U-40 insulin?
- Only with a conversion your vet has written down for you, and only if you do it every time. The scales differ by 2.5 times. The safe answer is to match the syringe to the insulin.
- What colour is a U-100 syringe cap?
- Orange. U-40 syringes have red caps. The colour is a check, not a substitute for reading the barrel.
- Is all human insulin in Australia U-100?
- Standard human insulin supplied in Australia is U-100. Some concentrated insulins exist at higher strengths and come in pens, not vials, so they are not drawn up with a syringe.