Second pass at: A reconstituted vial's beyond-use date and the reasoning behind common figures

Second pass at: A reconstituted vial's beyond-use date and the reasoning behind common figures 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 retatrutide and I am working to a 5.0 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.

This follows post #3 rather than contradicting it.

Protecting from light: amber vials or opaque containers reduce light exposure. If you are storing for long periods and light matters, this is worth doing. For short-term prepared solutions, ordinary storage away from direct sunlight is usually adequate.

Refrigeration slows degradation. Repeated warming and cooling is worse than continuous refrigeration. Light exposure is worth avoiding. The concentration matters because adsorptive losses are proportionally larger in dilute solutions. All of that is reasonably well supported.

Worth separating two things that post #5 runs together.

Vial integrity: once a vial is entered with a needle repeatedly, the rubber closure degrades and the integrity fails. Using the same vial repeatedly over weeks or months carries increasing risk of contamination. Single-use vials entered once are lowest risk.