Accepted answer
0.9 per cent is 9 mg of sodium chloride per millilitre — 0.9 g in 100 mL — which works out at about 154 mmol/L of each ion and is why it is called isotonic. For: it is isotonic, it is what a clinical setting reconstitutes into, and it introduces no benzyl alcohol. Against: at 9 mg/mL of chloride it is more ionic strength than plain water, which can shift the solubility of a peptide near its isoelectric point, and it is unpreserved, so repeated withdrawals have nothing protecting them. Across a course of withdrawals that second point dominates: the chemistry difference is small and arguable, the sterility difference is neither.
None of this is medical advice, and research-use-only material carries no regulatory approval.
The failure mode is bioburden accumulating faster than the benzyl alcohol suppresses it, which happens if the initial inoculum is large or the environment is filthy.
Concentration and unit conversion at a glance
| Vial | Diluent | Concentration | 0.25 mg | 0.5 mg | 1 mg | 2.5 mg |
|---|
| 5 mg | 1 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 5 mg | 2 mL | 2.5 mg/mL | 10 u | 20 u | 40 u | 100 u |
| 10 mg | 1 mL | 10 mg/mL | 2.5 u | 5 u | 10 u | 25 u |
| 10 mg | 2 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 10 mg | 3 mL | 3.33 mg/mL | 7.5 u | 15 u | 30 u | 75 u |
Units are U-100 insulin units, where 1 unit = 0.01 mL. Divide dose by concentration for millilitres, then multiply by 100.
The underlying point is that the additive suppresses Gram-positive bacteria, Gram-negative bacteria and fungi quite effectively, but it is not an adequate control for spore-forming organisms.
The 28-day beyond-use convention derives from USP standards, which base it on microbiological risk rather than on any one specific study.
If you are doing single-withdrawal use, sterile water is the better choice.