the bacteriostatic water thing finally clicked for me and I want to write it down
Posting this as a discussion rather than a claim: the bacteriostatic water 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 5mg vial. You add 3mL of bacteriostatic water. Concentration is mass over volume: 1.67mg/mL. A U-100 insulin syringe is graduated at 100 units per millilitre, so one unit is 0.01mL.
To draw 0.75mg you need 0.75 divided by 1.67 millilitres, which is 45 units on the barrel. Write "1.67mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
Wrote the date on the vial as well as the concentration after losing track of which one was opened first.
I will update this if the picture changes rather than quietly leaving it up.
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.
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.
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 sterile water, not bacteriostatic. Same appearance, different product, and it matters for a multi-draw vial.
What concentration did you actually reconstitute at, rather than what did you intend to?
Is that a U-100 syringe? The barrel markings change the answer completely.
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.
Left up and flaired Explainer — the numbers in this one check out.
I keep a laminated card in the drawer with the arithmetic on it. Looks ridiculous. Has never once let me draw the wrong volume.
If your calculation produces a fraction of a unit, the concentration is wrong for your dose. Change the water volume until the numbers land on whole units.
Pushback on the syringe advice: a U-100 barrel reads units of volume. Calling them "doses" is how the confusion starts.
do the arithmetic before you reconstitute, not while holding a syringe
label the vial with the date as well as the concentration
Shook the first vial because nobody told me not to. It was fine, but I have swirled every one since.
Show the arithmetic — where did the unit figure come from?
Correction: that gives 5 units, not what you have written. 30mg into 1mL is 30mg/mL and the division follows from there.
How many mg in the vial and how many mL are you adding?
if the maths gives you a fraction of a unit, change the concentration
more water is not more drug, it is the same drug in a bigger volume
swirl, never shake
Bacteriostatic or plain sterile water?
Spent 23 weeks confused about this until somebody drew the table for me. 20mg, 2mL, 10mg/mL, 2mg is 20 units. That is the whole thing.
I would not round that figure. At 2.5mg/mL a rounding error of two units is a real fraction of the intended amount.
reconstituting at a round concentration makes every later dose easy
Research-use-only material is not approved for human use, so treat everything in this thread as arithmetic about solutions rather than instructions for a person.
Correcting myself above: I wrote the concentration the wrong way up. Mass over volume, not volume over mass.
the powder cake sits at the bottom, the volume you add is the volume that counts
What is written on the vial right now?
Asked GGPeps what volume they reconstitute to for their own stability testing and they answered in one line. Useful thing to know.
- 1If your calculation produces a fraction of a unit, the concentration is…8 comments in this branch · started by u/adaeze_cabrera
- 2if the maths gives you a fraction of a unit, change the concentration8 comments in this branch · started by u/emil_wojcik