u/emil_zamora

do the arithmetic before you reconstitute, not while holding a syringe

How many mg in the vial and how many mL are you adding?

twice as much water halves the concentration and doubles the units for the same dose

twice as much water halves the concentration and doubles the units for the same dose

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.

Pushback on the syringe advice: a U-100 barrel reads units of volume. Calling them "doses" is how the confusion starts.

the number you write down is the number you will trust at 6am

I would not round that figure. At 20mg/mL a rounding error of two units is a real fraction of the intended amount.

do the arithmetic before you reconstitute, not while holding a syringe

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.

Reconstituted at 3mL when I meant to use a different volume. Caught it because the units looked wrong on the barrel. Now I write 3.33mg/mL on the glass before I put the cap back.

label the vial with the date as well as the concentration

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.

This. Showing the working is the fastest way for somebody else to spot the mistake.

What is written on the vial right now?

Bacteriostatic or plain sterile water?

Adding one thing — put the date on the label too. Concentration plus date and you can never mix two vials up.

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.

Are you accounting for the powder volume, or is that where the confusion is?

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.

twice as much water halves the concentration and doubles the units for the same dose

10mg into 1mL is 10mg/mL, and 2.5mg of that is 25 units

Correcting myself above: I wrote the concentration the wrong way up. Mass over volume, not volume over mass.

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.