mg vs mL: the 1,000× dosing error
Two units, three letters apart, no shared dimension. mg is mass; mL is volume. Confusing them is the single most common reason people inject 1,000× the intended dose — and the fix is a one-line habit.
Why mg and mL aren't interchangeable
A milligram is a mass — how much peptide. A milliliter is a volume — how much liquid. The bridge between them is concentration:
That means "0.5 mL" and "0.5 mg" are not the same dose unless the concentration happens to be exactly 1 mg/mL — which is uncommon. A 5 mg vial reconstituted with 2 mL is 2.5 mg/mL: 0.5 mL draws 1.25 mg, not 0.5 mg.
The 1,000× error pattern
The most common version: someone reads "1 mg" on the vial label, mentally substitutes "1 mL" when drawing, and pulls back a full mL — five to twenty times more peptide than intended. The math:
| Vial label | Reconstitution | Concentration | "1 mL" really delivers |
|---|---|---|---|
| 5 mg BPC-157 | 2 mL BAC water | 2.5 mg/mL | 2.5 mg (intended 0.25 mg → 10×) |
| 10 mg semaglutide | 2 mL BAC water | 5 mg/mL | 5 mg (intended 0.25 mg → 20×) |
| 20 mg tirzepatide | 2 mL BAC water | 10 mg/mL | 10 mg (intended 2.5 mg → 4×) |
The two-number habit
Write every dose with both fields. Never with just one.
Better: "0.25 mg semaglutide → draw 0.05 mL at 5 mg/mL"
The second form forces a concentration check every time. If the concentration is wrong, the volume is wrong, and you'll notice while writing it down — not after injecting.
Insulin syringes don't show mg
The U-100 insulin syringe most people use for subcutaneous peptide injection is graduated in "units" of volume. 100 units = 1 mL. The syringe does not know what's in the liquid. A syringe drawn to "10 units" pulls 0.1 mL — and how much peptide that contains depends entirely on the vial's concentration. See the companion post on why U-100 syringes show volume, not mass.
When the label is in mcg, not mg
Some compounded peptides and most insulin secretagogues label doses in micrograms (mcg). 1 mg = 1,000 mcg. A dose written as "250" with no unit is ambiguous and dangerous — it could be 250 mcg (0.25 mg) or 250 mg (a fatal overdose for most peptides). Always read the unit; never read the number alone.
A quick sanity check before injecting
- What's the target dose in mg or mcg? Write it down.
- What's the concentration in mg/mL? Calculate from vial size ÷ diluent volume.
- Required draw in mL = target mg ÷ concentration.
- Required units on U-100 syringe = mL × 100.
- Does the answer look like a normal dose for this peptide? If not, stop.
The reconstitution calculator does steps 2–4 automatically.
FAQ
Is a "10 unit" mark on a U-100 syringe the same as 10 mg?
No. 10 units on a U-100 syringe is 0.1 mL of volume. The mass of peptide in that volume depends on concentration. At 5 mg/mL it is 0.5 mg; at 2.5 mg/mL it is 0.25 mg.
Is mg the same as cc?
No. mg is mass. cc (cubic centimeter) is volume; 1 cc = 1 mL. Confusing mg and cc is the same 1,000× error.
My vial says "5 mg" with no mention of volume. What concentration is it?
You can't know until you reconstitute it. The label gives you the total mass in the vial; you choose the concentration by choosing how much diluent to add.
What if I already injected a 10× dose?
For most peptides the harm is concentration-dependent: severe nausea/vomiting on GLP-1s, hypotension/tachycardia on BPC-157, but rarely fatal at 10×. Call poison control, hydrate, monitor. Don't inject again until you've recalculated.
Related reading
Calculate every draw before you inject
Peptide Protocol's reconstitution calculator converts mg targets into the exact mL and U-100 unit count for any vial you log.
Get the iPhone app →Informational and educational only. Not medical advice. Consult a licensed clinician before starting, changing, or stopping any peptide protocol. Mentions of investigational, compounded, or research-use peptides are for informational purposes; many such substances are not FDA-approved for human use.