mg, mcg and units: the arithmetic people get wrong
Most dosing errors in this space are not judgement calls, they are unit conversions. What a "unit" on a syringe measures, how concentration turns milligrams into a mark on a barrel, and why your log should hold both.
Educational reference, not medical advice
The dose people talk about and the dose people measure are in different units, and the bridge between them is a number that is specific to the vial in front of you. That is the whole problem, and it is why a tenfold error is so easy to make: nothing about "20 units" tells you how many milligrams it is.
Three units, doing three different jobs
- Milligram (mg)
- A mass. One thousandth of a gram. GLP-1 doses are usually spoken about in these.
- Microgram (mcg or µg)
- A mass, one thousandth of a milligram. Most research peptides are dosed in these. 1 mg = 1000 mcg, and confusing the two is a factor of a thousand.
- Unit (U)
- A VOLUME, not a mass, when it is a mark on an insulin syringe. On a standard U-100 syringe, 100 units = 1 mL, so one unit is 0.01 mL. It says nothing at all about how much compound is in that volume.
That last one is the trap. "I take 20 units" is a complete description of a volume and an incomplete description of a dose. Two people can take 20 units of the same compound and be an order of magnitude apart, because their vials are at different concentrations.
Concentration is the bridge
Concentration is simply how much compound is dissolved in how much liquid: milligrams per millilitre. Once you have it, everything else is division.
- Concentration = total mg in the vial ÷ mL of liquid it is dissolved in.
- Volume for a dose = dose ÷ concentration.
- Syringe units = volume in mL × 100, on a U-100 syringe.
A worked example, using round numbers on purpose. A 5 mg vial reconstituted with 2 mL of liquid is at 2.5 mg/mL. A 0.5 mg dose is therefore 0.5 ÷ 2.5 = 0.2 mL, which is 20 units on a U-100 syringe. Change the liquid to 1 mL and the same vial is at 5 mg/mL: the same 0.5 mg dose is now 10 units. Same vial, same dose, half the marks on the barrel.
The four errors worth knowing about
- mg written where mcg was meant. A thousandfold error, and by far the most consequential one. If a compound is normally discussed in hundreds of mcg, a dose written as "250 mg" is a typo, not a protocol.
- Copying someone else's unit count. Units are volume. Another person's "12 units" is meaningless without their concentration, and forum posts almost never include it.
- Assuming every syringe is U-100. U-40 and U-50 syringes exist. On a U-40, 40 units is 1 mL, so the same mark is two and a half times the volume.
- Recording the volume instead of the dose. A log full of unit counts is unreadable the moment a vial is mixed differently, and it makes every level estimate wrong, because the estimator works in mass.
What to put in the log
Record the dose in mass, mg or mcg, as the primary field, because that is the number that stays true across vials, batches and years. Record the vial's concentration alongside it, so that the unit count you actually drew can be reconstructed and checked. Peptain stores both: you enter the vial once, and every dose after that is logged in milligrams while the app keeps track of what is left in the vial.
This matters beyond tidiness. Every level estimate is arithmetic on mass and time (see peptide half-life, explained), so a log kept in syringe units cannot produce one. And a prescriber reading "20 units" has to ask two questions before the entry means anything, which in practice means they will not ask.
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