Converting between mg/mL and units per dose, both directions — the long version

Converting between mg/mL and units per dose, both directions — the long version Writing it up because I had to work it out twice and would rather nobody else did.

Practical question with the units stated, because I have seen how quickly these go wrong without them.

I have a 2 mg vial of tirzepatide and I am working to a 1.0 mg step. My syringes are U-100 insulin syringes, 0.3 mL barrel.

I can do the arithmetic and I have done it twice, getting the same answer both times, but I would like someone to check the reasoning rather than the number — specifically whether I have thought about the residual volume correctly, and whether the graduation I am landing on is one a person can actually read.

Narrowing the opening post, because the general version has more than one answer.

Two people can reconstitute the same vial to different concentrations and both be right. The dose is the same; only the volume drawn differs. This confuses more discussions here than any other single point.

That is the shape of it. The detail is where I would expect to be corrected.

Dead volume is the part nobody mentions until it costs them a dose. A fixed-needle insulin syringe holds very little; a detachable-needle luer configuration can hold enough to matter at small doses.

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.

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.

I have written this out at length because the short version keeps being misread.

Post #5 and I disagree about the size of the effect, not about the direction.

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.

Not a strong opinion, just a consistent one.

That is a cleaner way of putting what I was circling around.

I read post #5 twice before replying, because I had assumed the opposite.

Over-dilution: if your target dose is 0.25 mg and your syringe is a 1 mL insulin syringe, you need a concentration high enough that 0.25 mg fits on the scale. A 0.25 mg/mL solution requires drawing the entire 1 mL syringe — not readable. A 5 mg/mL solution requires drawing 50 μL — also not practical on an insulin syringe.

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.

I had written a reply contradicting post #9 and deleted it. Here is what survived.

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.

I checked the source rather than the summary, and they differ.

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.

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.