Safety guide

How concentration and syringe markings work

Measurement mix-ups have caused serious overdoses with injectable products. This page explains the math, the mistakes, and how to check your work. It does not tell you what amount to use.

Converter

Enter your own numbers. The converter does arithmetic only; it does not suggest an amount and is not tied to any product.

Why this matters

FDA has received reports of adverse events, some requiring hospitalization, linked to dosing errors with compounded semaglutide. Most of the reports about measuring the dose described patients drawing up more than prescribed from a multiple-dose vial and taking five to 20 times the intended dose; many were unfamiliar with measuring a dose in a syringe and confused milliliters, milligrams and "units". FDA alert, July 2024.

The four numbers

1. Amount in the vial: How much drug the vial holds, usually in milligrams (mg). 1 mg = 1,000 micrograms (mcg or µg).
2. Total liquid in the vial: The total volume of solution, in milliliters (mL): for a dissolved powder, roughly the liquid added; for a ready-made solution, the volume and concentration on the label.
3. Concentration: Amount ÷ liquid, for example 5 mg ÷ 2 mL = 2.5 mg per mL. Change either number and every dose calculation changes.
4. Amount per dose: What one dose contains, in mg or mcg. Volume to draw = amount per dose ÷ concentration.

What syringe "units" are

Insulin syringe markings are calibrated for insulin: on a U-100 syringe, 100 units = 1 mL of U-100 insulin, so each unit marking is 0.01 mL. Used with any other liquid, the markings only tell you the volume, not how much drug is in it.
How much drug one unit holds depends entirely on the concentration: the same 10 units can hold very different amounts from two differently mixed vials.
U-40 syringes (common for pet insulin) mark 40 units per mL. Reading a U-40 syringe as if it were U-100 gives 2.5 times the volume.
"IU" or "international units" printed on hormones such as HGH or hCG measure biological activity. They are not syringe units.
U-100 syringes come in 0.3, 0.5 and 1 mL sizes; small syringes have finer markings and are easier to read accurately.

Worked example

A vial holds 5 mg. 2 mL of liquid is added, so the concentration is 5 ÷ 2 = 2.5 mg per mL (2,500 mcg per mL). On a U-100 syringe each unit is 0.01 mL, so each unit holds 2,500 ÷ 100 = 25 mcg. An amount of 0.25 mg (250 mcg) is 250 ÷ 25 = 10 units, or 0.1 mL. If the same vial had been mixed with 1 mL instead, the same 10 units would hold twice as much.

The mistakes that cause overdoses

mg and mcg mixed up: a 1,000-fold error.
mL and units mixed up: 1 mL is 100 units on a U-100 syringe, a 100-fold error.
Forgetting how much liquid went in, or using a new vial of a different size or concentration without recalculating.
Reading a dose written for one concentration (for example, a chart or a post) against a vial mixed differently.
U-40 versus U-100 syringes, or reading IU on a label as syringe units.
Very small volumes: the smaller the volume, the more a small reading error changes the dose.

Checks before anything is drawn up

Write the concentration on the vial when it is mixed (for example "2.5 mg/mL").
Recalculate every time the vial, the liquid volume or the syringe changes.
Have a pharmacist or clinician check the math, the concentration on the label and the syringe type.
Remember that the math is only as good as the label: products sold online as "research peptides" are not FDA approved, and their contents are not verified.

This guide is general safety information about measurement, not medical advice, and does not recommend any product or amount. Doses shown elsewhere on this site are what studies used, not instructions. See the terms.