Converting between mg/mL and units per dose, both directions

Do not shake. Swirl, or leave it. Vigorous agitation introduces air and shear, and neither helps a peptide go into solution any faster than patience does.

Post #13 is right about the mechanism and I think understates the practical bit.

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.

If the premise is wrong, everything after it is decoration.

The decimal-point error: computing 5 mg / 2 mL as 0.25 mg/mL instead of 2.5 mg/mL is the most common arithmetic error in this subcategory. The habit that catches it: writing the units in every step of the calculation.

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

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.

Do not shake. Swirl, or leave it. Vigorous agitation introduces air and shear, and neither helps a peptide go into solution any faster than patience does.

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.

Nothing to add on the substance. Thank you for taking the question at face value.

Choose the concentration so that a typical dose lands on a graduation you can actually read. A dose that falls between two marks on a 1 mL barrel is a dose you will get wrong sooner or later.

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.

Everything in post #20 holds. The case it does not cover is the one I have.

Do not shake. Swirl, or leave it. Vigorous agitation introduces air and shear, and neither helps a peptide go into solution any faster than patience does.

If this contradicts something upthread, the upthread version may well be the better one.

Building on post #22 rather than restating it.

Reconstituting a multi-strength kit: if a kit contains 5 mg, 10 mg, 15 mg vials and you are reconstituting all of them, writing the concentration on each vial in permanent marker as you go is the single most useful thing you can do to avoid dose errors later.

Flagging that the sources on this are thinner than the confidence in the thread suggests.

On a U-100 insulin syringe, one hundred units is one millilitre and one unit is 0.01 millilitres. Units are volume marks. They mean nothing until you know the concentration.

Genuinely open to being wrong about this one.

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.

Do not shake. Swirl, or leave it. Vigorous agitation introduces air and shear, and neither helps a peptide go into solution any faster than patience does.

Arithmetic step by step: a 5 mg vial with 2 mL of diluent gives (5 mg) / (2 mL) = 2.5 mg/mL. On a U-100 syringe at that concentration, 100 units = 1 mL = 2.5 mg, so each unit = 0.025 mg. A 0.25 mg dose = 0.25 / 0.025 = 10 units. Different concentration: different arithmetic, same principle.

I am confident about the direction and much less about the magnitude.

On post #28 — agreed on the reasoning, with one qualification.

Do not shake. Swirl, or leave it. Vigorous agitation introduces air and shear, and neither helps a peptide go into solution any faster than patience does.

If that is already documented somewhere, ignore me and link it.