the dead space thing finally clicked for me and I want to write it down
Posting this as a discussion rather than a claim: the dead space thing finally clicked for me and I want to write it down.
Why "more water" keeps confusing people, including me for a month.
The vial contains a fixed amount of peptide. Water changes nothing about that amount. What it changes is concentration, and concentration is the divisor between the dose you want and the volume you draw.
So doubling the water halves the concentration and doubles the units for the same dose. Nothing is diluted in any sense that matters; you are simply reading a bigger number off the same barrel. Pick the water volume that puts your usual dose on a whole number of units and life gets easier.
The whole calculation, once, with real numbers.
You have a 20mg vial. You add 1mL of bacteriostatic water. Concentration is mass over volume: 20mg/mL. A U-100 insulin syringe is graduated at 100 units per millilitre, so one unit is 0.01mL.
To draw 5mg you need 5 divided by 20 millilitres, which is 25 units on the barrel. Write "20mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
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.
That is everything I have. The rest is opinion and I have tried to keep it out.
best — the order this archive was captured in
Bacteriostatic water contains benzyl alcohol as a preservative, which is what makes repeated entry into the same vial reasonable. Plain sterile water has no preservative.
Left up and flaired Explainer — the numbers in this one check out.
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.
Left up and flaired Explainer — the numbers in this one check out.
Disagreeing with this line only: adding more water is not a safety margin, it is a different divisor.
Disagree with the "more water is safer" framing. It is not safer, it is just a different number to divide by.
That is sterile water, not bacteriostatic. Same appearance, different product, and it matters for a multi-draw vial.
That is sterile water, not bacteriostatic.
Right, and the arithmetic behind it: 20mg in 2mL is 10mg/mL, so 2.5mg is 25 units.
more water is not more drug, it is the same drug in a bigger volume
Correction: that gives 40 units, not what you have written. 5mg into 1mL is 5mg/mL and the division follows from there.
The dead space question sent me down a rabbit hole for a week. It is a fraction of a unit and it is the same every draw.
That only works if the fill weight is exactly the label weight, and it usually is not exactly anything.
reconstituting at a round concentration makes every later dose easy
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.
do the arithmetic before you reconstitute, not while holding a syringe
the number you write down is the number you will trust at 6am
Yes. Writing 10mg/mL on the vial takes four seconds and removes the entire class of error this board exists to fix.
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.
10mg into 2mL is 5mg/mL, and 0.75mg of that is 15 units
if the maths gives you a fraction of a unit, change the concentration
Agreed on the swirl. There is no reason to shake it and a good reason not to.
aim the stream at the glass wall, not at the cake
label the vial with the date as well as the concentration
bacteriostatic water has the preservative, sterile water does not
What is written on the vial right now?
swirl, never shake
I would not round that figure. At 5mg/mL a rounding error of two units is a real fraction of the intended amount.
Not quite. Bacteriostatic and sterile water are not interchangeable for a multi-draw vial, and the difference is the preservative.
- 1Left up and flaired Explainer — the numbers in this one check out.11 comments in this branch · started by u/bac_water_bandit
- 2Yes. Writing 10mg/mL on the vial takes four seconds and removes the entire…6 comments in this branch · started by u/andres_sorensen