Details up front: 8 mg · 2 mL.
I can do the algebra. I am not confident about the conversion factors.
If there is a standard way to lay this out, I would rather learn that than invent one.
Can someone show the working rather than just the answer?
4 mg/mL, so one unit carries 0.04 mg. 8 ÷ 2 = 4 mg/mL; one unit on a U-100 barrel is 0.01 mL; 4 × 0.01 = 0.04 mg per unit. To go the other way, divide your intended dose by 0.04: a 0.4 mg dose is 10 units, and a 0.8 mg dose is 20. Write both the concentration and the milligrams per unit on the vial.
The arithmetic only stops being confusing once you work it through once and see that it is straightforward.
Rotation of injection site is a tolerability measure, not a pharmacokinetic one, but if you are going to do it you might as well do it right.
On filtration: a 0.22 µm syringe filter will remove particulates and organisms, and it will also adsorb a fraction of your peptide onto the membrane — with a low-binding PVDF or PES membrane the loss is typically a few per cent.
The insulin-unit standard U-100 means 100 units per millilitre, so one unit is 0.01 mL — this is the conversion that trips up more people here than any other single piece of arithmetic.
I would flag the obvious failure mode: people get the concentration right, get the volume right, and then read the syringe against the wrong scale.
Write the arithmetic on the vial label. It costs nothing and removes the step where you reconstruct it from memory.
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