U-40 vs U-100 Insulin Syringes: Conversion, Differences & Which to Use

What U-40 and U-100 actually mean

The number is the insulin concentration the syringe was designed for: U-100 means 100 units of insulin per ml, U-40 means 40 units per ml. On a U-100 syringe, 1 unit of scale = 0.01 ml. On a U-40 syringe, 1 unit of scale = 0.025 ml. The barrels look similar; the scales are not.

U-100 is the human standard worldwide and what every peptide protocol assumes. U-40 syringes exist mainly for veterinary insulin (dogs and cats), and they still turn up in pharmacies and online orders — which is where the confusion starts.

Converting between them

Convert through milliliters. U-100 units ÷ 100 = ml. U-40 units ÷ 40 = ml. So 10 U-100 units = 0.10 ml = 4 U-40 units, and 10 U-40 units = 0.25 ml = 25 U-100 units. Rule of thumb: a U-40 reading is 2.5× the U-100 reading for the same volume.

Quick table (same liquid volume): 0.1 ml = 10 U-100 = 4 U-40. 0.25 ml = 25 U-100 = 10 U-40. 0.5 ml = 50 U-100 = 20 U-40. 1 ml = 100 U-100 = 40 U-40.

Why the mix-up matters

Peptide dosing is written in U-100 units almost everywhere. Drawing 'to 20 units' on a U-40 syringe delivers 0.5 ml — 2.5× the volume, 2.5× the dose. In the other direction, using a U-100 syringe while following a U-40 vet instruction under-doses by the same factor. For drugs like insulin this has caused hospitalizations; for peptides it silently wrecks a protocol.

Which to buy for peptides

U-100, always. Choose the barrel by dose volume: 0.3 ml (30 units, half-unit marks) for small doses under 0.3 ml, 0.5 ml for mid-range, 1 ml for doses above 50 units or large-volume compounds like NAD+ and glutathione. Check the printed 'U-100' on the packaging before the first draw, and if a syringe shows a 40-unit scale, put it aside.