mg, mL and IU
Three abbreviations show up on almost every peptide vial and forum post: mg, mL, and IU. They look like they belong to the same family. They don't. One measures weight, one measures volume, and one measures how strongly something works. Mixing them up is behind a huge share of dosing questions, so let's pull them apart properly.
mg — the weight of the actual drug
A milligram is a unit of mass. Nothing more exotic than that. When your vial says 5 mg, it means there are five milligrams of the peptide sitting in there as dry powder, full stop. It says nothing about liquid, because at that point there isn't any.
You'll also see micrograms, written mcg or µg. A milligram is a thousand of them: 1 mg = 1000 mcg. Most peptides get dosed in the hundreds of micrograms, which is why you see "250 mcg" far more often than "0.25 mg", even though they're the same amount.
The thing to hold onto: mg is the drug itself. It's the number that decides what the compound does in your body. Everything else on this page is just packaging around it.
mL — the volume of liquid
A milliliter is a unit of volume. It answers "how much fluid", not "how much drug". When you reconstitute a peptide, you add bacteriostatic water, and the amount you add is measured in mL. Two mL, one mL, three mL, whatever you chose.
Here's the part that surprises people. The volume you add doesn't change how much drug is in the vial. Pour in 1 mL or pour in 3 mL, those 5 mg of peptide are still 5 mg. All you've changed is how spread out they are. More water simply thins the same drug across a bigger pool of liquid.
That relationship has a name: concentration. Milligrams of drug divided by milliliters of water gives you mg/mL, and that's the single number your syringe math runs on. If you want that walked through slowly, how to reconstitute peptides does exactly that.
IU — a measure of activity, not weight
International Units are the odd one out, and the reason people get tangled.
An IU doesn't measure mass at all. It measures biological activity — how much of an effect a substance produces — against an agreed international reference standard. It exists because for some older biologics, two batches with the same weight didn't necessarily do the same amount of work. Potency could vary. So instead of weighing the compound, they measured what it actually did and called one unit of that effect an International Unit.
Because it's defined per substance, there's no universal "IU to mg" conversion. Each one has its own. The factor for growth hormone is nothing like the factor for, say, insulin, and inventing a general rule would just get you into trouble. In practice only a handful of things get dosed this way — growth hormone, HCG, insulin, a few vitamins. The overwhelming majority of research peptides are dosed by weight in mcg or mg, and never touch IU at all.
The trap: syringe "units" are not IU
Now the mix-up that actually causes harm. Look at a standard U-100 insulin syringe and you'll see it marked in "units", 1 to 100. Those units are volume marks. On a U-100 syringe, 100 units is 1 mL, so a single unit is 0.01 mL. That's it. They tell you how far to pull the plunger, nothing else.
They have nothing to do with International Units. The word "unit" doing double duty is just an unfortunate accident of language. One is a line on a barrel that measures liquid; the other is a measure of biological potency. Reading "10 units" on a syringe and thinking you've drawn "10 IU" of something is how people end up wildly off, in either direction.
So when a protocol says "pull to 15 units", it means 0.15 mL of your solution. What that 0.15 mL actually delivers in drug depends entirely on your concentration — back to mg/mL again.
Putting it together
Say you've got a 10 mg vial and you add 2 mL of bacteriostatic water.
Concentration is 10 ÷ 2 = 5 mg/mL. You want a 500 mcg dose, which is 0.5 mg. Volume needed is 0.5 ÷ 5 = 0.1 mL, and on the syringe that's 10 units. Three different units, one clean answer, and not an IU in sight.
If you'd rather not do this by hand every time, the reconstitution calculator takes the vial strength, your water volume and your target dose and just tells you the mark to pull to. And once the dosing is sorted, the drug-level plotter shows how the level rises and falls between shots.
Get these three straight and the rest of peptide math stops feeling like a foreign language. mg is what the drug weighs. mL is how much liquid it's in. IU, on the rare occasion it applies, is how hard it works — and it is never the same thing as the units printed on your syringe.
This is educational information, not medical advice. Dosing any medicine belongs with a clinician who is supervising you.