How to read an insulin syringe for peptides

An insulin syringe is marked in units. On a U-100 syringe, 100 units equals 1 mL, so 1 unit is 0.01 mL. That is all a unit means here: a measure of liquid.

How much peptide is in those units depends on how the vial was mixed. Twenty units is always 0.20 mL. It might hold 500 mcg in one vial and 2 mg in another. Syringe units are also not International Units (IU), which measure biological activity for products like growth hormone.

FDA has documented overdoses from exactly this confusion with compounded semaglutide, including a 0.25 mg dose (5 units) prescribed as 25 units.

U-100 syringes come in three common sizes. The scale is the same on all of them; only the capacity and the line spacing change.

• 1 mL (100 units): numbered every 10 units. On most brands each small line is 2 units, so odd numbers sit between lines.
• 0.5 mL (50 units): numbered every 10 units, a line for every unit.
• 0.3 mL (30 units): numbered every 5 units, a line for every unit. Some models add half-unit lines.

You will see these sold as "U-100 1 mL", "U-100 0.5 mL" (sometimes called a 50-unit or U-50 syringe) and "U-100 0.3 mL" (a 30-unit syringe). They all measure 0.01 mL per unit.

Pick the smallest barrel that holds your dose. The lines sit further apart, so small doses are easier to hit.

U-40 syringes exist for veterinary insulin, where 40 units equals 1 mL. A unit on a U-40 syringe is 2.5 times more liquid than a unit on a U-100 syringe. The maker of one veterinary insulin tells owners to use only U-40 syringes with it for exactly that reason.

Peptide math on this site always assumes U-100. Check the barrel or the box for "U-100" before you trust any unit count.

Read the barrel and the packaging. Confirm it says U-100 and note the capacity: 1 mL, 0.5 mL or 0.3 mL. That tells you how many units it holds and roughly what each line is worth.

Find two numbered marks, count the small lines between them, and divide. On a 1 mL barrel numbered 10, 20, 30 with four lines between each number, each line is 2 units. On a 0.5 mL barrel with nine lines between 10 and 20, each line is 1 unit.

Do this once for every new box. Brands differ.

Units = dose in mg divided by concentration in mg per mL, times 100.

If your vial is 2.5 mg per mL and your dose is 0.25 mg, that is 0.10 mL, or 10 units. Let the reconstitution calculator do this for your vial, and see mg to units conversion for the full method.

1. Swab the vial stopper.
2. Pull the plunger to your unit mark to fill the syringe with that much air, push the needle into the vial and inject the air.
3. Turn the vial upside down, keep the needle tip in the liquid, and draw a little past your mark.
4. Tap the barrel so bubbles rise, push them back into the vial, and stop exactly on your line.

Hold the syringe at eye level. The dose is where the top edge of the plunger's rubber stopper (the edge nearest the needle) meets the scale. Do not read from the bottom of the rubber or from the tip of the cone.

Bubbles take up space that should be liquid. A big bubble in the barrel means a short dose, so clear it and re-check.

Insulin syringes have a fixed needle. Higher gauge means thinner: 31G is finer than 29G. Expert insulin guidance favors the shortest needles, with 6 mm as the first choice on syringes, because they reliably reach the fat layer without going into muscle. Peptide guides commonly suggest 30 or 31 gauge at 5/16 inch (8 mm).

Fixed needles also mean very little dead space, so the dose you see is close to the dose you get.