May 22, 2026
U-100 vs U-40 Syringes — How to Read Them
The most common dosing error in peptide protocols is reading a U-100 scale as if it were U-40 (or vice versa). Here's how to never make it.
The most common dosing mistake in self-administered peptide protocols isn't math — it's reading the wrong syringe scale. This post explains the difference and how to never confuse the two.
The short version
- U-100 = 100 units per 1 mL
- U-40 = 40 units per 1 mL
- The "unit" is a scale convention, not a fixed amount of drug.
A 0.5 mL injection reads as 50 units on a U-100, 20 units on a U-40. Same volume, same drug, different number on the barrel.
How to identify which one you have
- U-100 1 mL syringe — barrel says "U-100", max is 100.
- U-100 0.5 mL syringe — barrel says "U-100", max is 50.
- U-100 0.3 mL syringe — max is 30.
- U-40 1 mL syringe — barrel says "U-40", max is 40.
Default to caution: if you can't confirm, don't use it for peptides.
The math
| Inject | U-100 units | U-40 units |
|---|---|---|
| 0.10 mL | 10 | 4 |
| 0.20 mL | 20 | 8 |
| 0.25 mL | 25 | 10 |
| 0.50 mL | 50 | 20 |
| 1.00 mL | 100 | 40 |
Worked example
5 mg vial + 2 mL BAC water → 2.5 mg/mL. Target 0.5 mg → volume 0.2 mL.
- U-100 syringe: draw to 20 units ✓
- U-40 syringe: draw to 8 units ✓
- Wrong: "20 units" on U-40 = 0.5 mL = 1.25 mg — 2.5× the intended dose.
Which should you use?
For most subcutaneous peptide protocols (GLP-1s, BPC-157, ipamorelin) where typical doses are below 1 mL, U-100 has finer gradations (1-unit lines) for better readability.
- Can I use a "tuberculin" syringe with mL marks?
- Yes. Inject the same volume; read mL directly.
- What if I only have U-40 but my protocol assumes U-100?
- Compute volume in mL first, then multiply by 40. Don't try to convert unit-to-unit.
Informational only. Not medical advice.