unpopular opinion: most of what gets said here about concentration is guesswork
unpopular opinion: most of what gets said here about concentration is guesswork. Change my mind, genuinely — I have no stake in being right about this.
The failure modes I have actually seen posted here, in order of frequency.
One: reconstituting at a different volume than intended and not writing it down, so the next draw is based on a remembered concentration. Two: treating units as a dose rather than a volume, which breaks the moment the concentration changes. Three: assuming a syringe is U-100 when it is not.
All three are solved by the same two habits — do the arithmetic before the water goes in, and write the result on the glass. Neither costs anything and between them they cover almost every question this board gets.
Shook the first vial because nobody told me not to. It was fine, but I have swirled every one since.
The whole calculation, once, with real numbers.
You have a 15mg vial. You add 3mL of bacteriostatic water. Concentration is mass over volume: 5mg/mL. A U-100 insulin syringe is graduated at 100 units per millilitre, so one unit is 0.01mL.
To draw 1.5mg you need 1.5 divided by 5 millilitres, which is 30 units on the barrel. Write "5mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
Ask me anything specific. Anything general I will probably get wrong.
best — the order this archive was captured in
The lyophilised cake displaces a small volume, so the true concentration is very slightly lower than the arithmetic suggests. It is a fraction of a percent and it is dwarfed by fill tolerance.
Agreed on the swirl. There is no reason to shake it and a good reason not to.
twice as much water halves the concentration and doubles the units for the same dose
Reminder: this board does maths, not dosing. Nobody here tells another member what amount to take.
more water is not more drug, it is the same drug in a bigger volume
Small fix — a U-100 syringe is 100 units per mL. If yours is marked differently it is not U-100 and the whole calculation changes.
Small fix — a U-100 syringe is 100 units per mL.
This is the sentence that makes the rest unnecessary. Write it on the vial and the problem is gone.
Careful — that arithmetic is wrong. 5mg in 2mL is 2.5mg/mL, so 1mg draws to 40 units, not what you wrote.
What is written on the vial right now?
This. Showing the working is the fastest way for somebody else to spot the mistake.
What concentration did you actually reconstitute at, rather than what did you intend to?
Show the arithmetic — where did the unit figure come from?
The arithmetic in full: concentration is mass over volume, so 20mg into 1mL gives 20mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 5mg is 5 divided by 20 millilitres, which is 25 units.
That is sterile water, not bacteriostatic. Same appearance, different product, and it matters for a multi-draw vial.
Did the maths give you a whole number of units, or a fraction?
The arithmetic in full: concentration is mass over volume, so 20mg into 1mL gives 20mg/mL.
aurel_boateng has the units point exactly right. They are a volume on the barrel and nothing else.
Wrote the date on the vial as well as the concentration after losing track of which one was opened first.
bacteriostatic water has the preservative, sterile water does not
Adding more water does not change how much peptide is in the vial. It changes the concentration, which changes the volume you draw for the same amount. That is the entire relationship.
- 1Reminder: this board does maths, not dosing. Nobody here tells another…9 comments in this branch · started by u/units_not_mg