Insulin syringes are the standard tool for injecting compounded GLP-1 medications like semaglutide and tirzepatide. But they were designed for insulin — a drug dosed in units with a fixed, standardized concentration. GLP-1 medications are dosed in milligrams from vials that vary in concentration. That mismatch is the source of most syringe-reading confusion.
This article explains exactly what the unit markings on an insulin syringe mean, why they exist, how they relate to milliliters, and how to use them accurately when drawing a GLP-1 dose.
What "Units" on a Syringe Actually Mean
The unit markings on an insulin syringe are a fixed volumetric scale — not a medication scale. One unit on a U-100 syringe always equals 0.01 mL, regardless of what liquid is inside. The "U-100" designation means the syringe was calibrated assuming a concentration of 100 units per mL, which is the standard concentration for human insulin.
When you use a U-100 syringe to draw a GLP-1 medication, the unit markings still represent volume. You are simply using the syringe as a precision measuring tool, not as an insulin delivery device. The math still works — you just have to calculate which unit mark corresponds to your target mL volume.
1 unit = 0.01 mL | 10 units = 0.10 mL | 100 units = 1.00 mL
U-100 vs U-40: Understanding the Difference
Two syringe types are commonly available: U-100 and U-40. They look similar but their scales are different. A U-40 syringe holds 40 units per mL instead of 100. Drawing to the same unit marking on each syringe delivers a different physical volume — and therefore a different dose.
Standard for most compounded GLP-1 use. Orange cap in the US. Most compounding pharmacy instructions assume this type.
Less common. Red cap in some markets. Used for veterinary insulin and in some countries as a standard. Using this with U-100 instructions produces a significant dosing error.
Unless your pharmacy or prescriber specifies otherwise, assume U-100. If you are unsure, check the syringe packaging — it will state U-100 or U-40 clearly on the label.
How to Convert Between Units and mL
For a U-100 syringe, the conversions are straightforward multiplications:
Units = mL × 100
mL = Units ÷ 100
For a U-40 syringe, substitute 40 in place of 100 in both formulas. The physical volume of liquid drawn is what matters for dosing — and that volume must match your calculated target regardless of which scale you read it from.
How to Read Syringe Markings Accurately
- Identify the syringe capacity. Common sizes are 0.3 mL (30 units), 0.5 mL (50 units), and 1.0 mL (100 units) for U-100 syringes. Choose a syringe whose maximum capacity is at least as large as the volume you need to draw.
- Locate the major and minor tick marks. Most U-100 syringes label every 5 or 10 units with a number, and place unmarked ticks between them at 1- or 2-unit intervals. Identify how many units each small tick represents on your specific syringe.
- Read the bottom of the plunger stopper. The flat, lower edge of the rubber plunger stopper is the reference line. Align this edge — not the dome or the top edge — to your target marking.
- Check from eye level. Tilt or hold the syringe so you are looking directly across the barrel at the marking, not at an angle. Parallax error — reading a tilted syringe — is a common source of small but consistent measurement error.
- Double-check by converting back to mL. After drawing, confirm the unit reading matches your expected mL volume using the conversion formula. If they do not align, investigate before injecting.
Reference Tables
U-100 syringe: units to mL conversion
| Units (U-100) | Volume (mL) | Typical GLP-1 Use Case |
|---|---|---|
| 5 units | 0.05 mL | Very low starting dose (e.g. 0.25 mg at 5 mg/mL) |
| 10 units | 0.10 mL | Low dose at higher concentration |
| 20 units | 0.20 mL | 0.5 mg at 2.5 mg/mL; 1.0 mg at 5 mg/mL |
| 25 units | 0.25 mL | 0.25 mg at 1 mg/mL |
| 40 units | 0.40 mL | 1.0 mg at 2.5 mg/mL; 2.0 mg at 5 mg/mL |
| 50 units | 0.50 mL | 0.5 mg at 1 mg/mL |
| 100 units | 1.00 mL | 1.0 mg at 1 mg/mL (max capacity of a 1 mL syringe) |
Same unit marking — different volumes by syringe type
| Syringe Reading | U-100 Volume | U-40 Volume | Difference |
|---|---|---|---|
| 10 units | 0.10 mL | 0.25 mL | +150% |
| 20 units | 0.20 mL | 0.50 mL | +150% |
| 25 units | 0.25 mL | 0.625 mL | +150% |
| 40 units | 0.40 mL | 1.00 mL | +150% |
| 50 units | 0.50 mL | 1.25 mL | +150% |
Using a U-40 syringe when U-100 is assumed always results in drawing 2.5 times the intended volume — a substantial overdose at any GLP-1 dose level.
Syringe capacity guide by dose volume
| Syringe Size | Max Volume | Max Units (U-100) | Best For |
|---|---|---|---|
| 0.3 mL syringe | 0.30 mL | 30 units | Very small doses; highest per-tick precision |
| 0.5 mL syringe | 0.50 mL | 50 units | Low-to-mid doses; good precision |
| 1.0 mL syringe | 1.00 mL | 100 units | Higher dose volumes; more flexible |
Need help calculating your dose?
Use the GLP-1 dosage calculator.
Common Mistakes
Assuming units equal milligrams. Units on an insulin syringe measure volume, not medication amount. A reading of 25 units does not mean 25 mg — it means 0.25 mL. The milligram dose you receive depends on both the volume drawn and your vial's concentration.
Using a U-40 syringe with U-100 instructions. This is the most consequential syringe error possible. A U-40 syringe delivers 2.5 times the volume for any given unit marking compared to a U-100. If your instructions were written for U-100 and you use U-40, you will draw — and inject — 2.5 times your intended dose. Always confirm syringe type before drawing.
Reading the top of the plunger stopper instead of the bottom. The rubber plunger stopper has a dome-shaped top and a flat bottom edge. The correct reference point is the flat bottom edge. Reading the top consistently adds a small but real volume to every draw.
Choosing a syringe that is too large for the dose. A 1 mL syringe used to draw a 0.05 mL dose requires reading a marking near the very bottom of the barrel where tick marks are spread far apart. A 0.3 mL syringe gives you finer gradations across the same draw — improving accuracy significantly at small volumes.
Misreading between-tick values. Some syringes mark every 2 units rather than every 1. If you need to draw to an odd number — say 15 units — on a syringe marked in 2-unit increments, you must estimate the midpoint between ticks. When precision matters at small volumes, a finer-graduated syringe eliminates this guesswork.
Frequently Asked Questions
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