Which syringe matches my insulin?

What the U number means

Insulin strength is printed on the vial and the box as U-40 or U-100. U-40 means 40 units per ml. U-100 means 100 units per ml. Many vet insulins are U-40, while many human insulins and most pharmacy syringes are U-100, so it's easy for the two to end up side by side in your fridge or kitchen drawer.

Each syringe is made for one strength only, and it says U-40 or U-100 on the barrel and on the packet. U-40 syringes often have a red cap and U-100 syringes an orange one, but colours vary by maker and country. Always read the printed U number. Never go by the colour of the cap.

Why a mismatch matters

The lines on a syringe only measure correctly with insulin of the same strength. U-40 insulin drawn into a U-100 syringe gives your dog too little, about 2.5 times less than you meant. U-100 insulin drawn into a U-40 syringe gives too much, about 2.5 times more, and that can cause a dangerous low.

Please don't try to work out a conversion yourself, or copy one from another owner. If the U numbers don't match, get the matching syringe before the next injection.

Before every injection

• Read the vial or box label: U-40 or U-100?
• Read the syringe packet and barrel: U-40 or U-100?
• Check the vial and the syringe show the same U number, every time.
• If they don't match, stop. Don't inject. Call your vet or chemist and ask for the right syringe.

If your dog uses an insulin pen

Insulin pens work differently. They use their own cartridges and their own pen needles. Only use the pen and cartridge your vet prescribed, and ask your vet before swapping either one. For needle thickness and length, see our page on choosing syringes and needles.

What to write down

• The bottle label
• The syringe box and wrapper
• When the possible mistake happened and how your dog seems

If you think a mismatch happened

This is an emergency, even if your dog seems fine right now. Call your vet or an emergency clinic straight away, and have the vial and the syringe packet in front of you. Don't try to correct it yourself: don't give more insulin, and don't give a guessed amount of sugar. If your dog is weak, trembling, collapses or has a seizure, get emergency care now.

Mix-ups happen to careful owners, often when a new box of syringes comes from a different shop. Once it's sorted, ask your vet to write down the exact syringe that matches your dog's insulin, and keep that note with your supplies.