Dr.RaviCardio said:

Dead space is the answer to the missing dose.

I am going to disagree with reconstituting low as a general rule. More diluent, more punctures, more in-use days at room temperature, and the stability trade-off is real. Precision is not the only variable being optimised.

fiona_glasgow said:

I reconstituted a 10mg vial with 2ml of bacteriostatic water and I want someone to check my unit arithmetic before I draw anything, because I keep…

Peptide calculator app recommendation for dosing arithmetic: I use PeptideCalc to calculate reconstitution volumes and injection doses. Eliminates math errors completely.

Input: vial size (mg), desired concentration (mg/mL), prescribed dose (mg)
Output: BAC water volume (mL), injection volume (units on insulin syringe)

A math error with injectable medication can mean underdosing or overdosing. Use a calculator every time until the math becomes second nature.

TrialNerd_Beth said:

I am going to disagree with reconstituting low as a general rule.

Reconstitution tip for anyone mixing dosing arithmetic from lyophilized powder: use bacteriostatic water, NOT sterile water. The benzyl alcohol in BAC water preserves multi-dose vials for up to 28 days.

Inject the water slowly down the side of the vial. NEVER shake — gentle swirling only until fully dissolved. Should be crystal clear with no particles.

Following on from mike_nyc — and this may be the naive question:

How much material I am losing to dead space, and whether that explains why a 10mg vial gives me nine usable draws rather than ten?

Closing the loop on my own question.

My error was treating units as a mass. Once I wrote mg/ml on the vial in marker and worked in millilitres first, the two of us got the same answer.