A 5 mg vial and a 0.25 mg dose: the arithmetic in full — what changed since

Worked example, since the arithmetic is the whole question. Five milligrams into one millilitre is 5 mg/mL. A 0.25 mg dose is 0.05 mL, which is five units on a U-100 syringe. Check that against your own numbers rather than taking mine.

How much of the diluent volume the powder itself displaces: for a small peptide vial, the powder volume is negligible. For a larger vial or a kit with multiple compounds, the displacement can be a few tenths of a millilitre. If precision matters to you, account for it by targeting a final weight rather than a final volume.

I have changed my mind on this once already, so take it as current rather than settled.

Filter needles are worth considering if you are drawing from a glass ampoule and are pointless overhead for a stoppered vial. The trade is dead volume against particulate risk.

Correct me on the arithmetic if it is wrong; I would rather know.

Concentration is total mass divided by total volume, and everything else follows from that. Ten milligrams into two millilitres is five milligrams per millilitre, and it does not matter how the vial was labelled before you added anything.

It is the kind of thing that is obvious once and never again.

Worked example, since the arithmetic is the whole question. Five milligrams into one millilitre is 5 mg/mL. A 0.25 mg dose is 0.05 mL, which is five units on a U-100 syringe. Check that against your own numbers rather than taking mine.

Scoping that to what I have actually seen rather than what I have read.

On post #57 — agreed on the reasoning, with one qualification.

Preservative effectiveness is tested against a defined microbial challenge under defined conditions. It is not a licence to treat an entered vial as sterile indefinitely, and no supplier claims otherwise.

Genuinely open to being wrong about this one.

Choosing a concentration on purpose rather than by accident: starting with "I want to draw 0.5 mL per dose" and working backward to the required concentration is more efficient than picking a diluent volume and hoping the math works out. State your target volume, then the required concentration follows.

I have separated what I observed from what I concluded, which does not always happen.

Second-hand on 5 mg vial, so weight it accordingly — someone whose method I trust told me this and I have not verified it myself.

Where I part company with post #82, and it is a narrow parting.

Where the 5 mg vial reasoning breaks down for me is the step from the group result to the individual case. That step is almost never argued for.