Myth Correction

U-100 vs U-40 — the 2.5× error

More units marked on a barrel is not more medication. It depends entirely on which syringe you're holding.

The mix-up

U-100 syringes are graduated assuming 100 units per milliliter. U-40 syringes are graduated assuming 40 units per milliliter. The barrels look similar at a glance. The scale printed on them is not interchangeable.

Draw to the same numbered line on the wrong syringe type, and the actual volume drawn is off by 2.5×. That's not a rounding error — it's a category of dosing mistake documented in real clinical literature on insulin administration, well outside this niche too.

Why this is under-taught: the syringes look similar enough that the error is easy to make and easy to miss. Nobody double-checks a tool they assume they already understand.

The rule

Match the syringe type to what you're calculating for, every time — not from memory, not from habit. Check the barrel markings before you draw, not after.

We don't give dosing instructions. We do insist people understand the tools they're using well enough to catch this kind of error themselves — see why we don't answer dosing questions.

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