Vials and reconstitution

The vial tracker does two jobs. First, it does the reconstitution maths: you enter the amount of peptide in the vial and the volume of bacteriostatic water you added, and the app returns the concentration and the number of units to draw for the dose you want, on a U-100 or a U-40 syringe. Second, it keeps track of each vial after that: a running count of doses left, an expiry date, and a refill reminder when three doses remain.

How it works, step by step

1. Enter the vial. The amount of peptide on the label, for example 5 mg.
2. Enter the water. The volume of bacteriostatic water you added, for example 2 mL.
3. Read the concentration. 5 mg in 2 mL is 2.5 mg per mL.
4. Enter the dose. The app returns the volume and the syringe units to draw, in U-100 and U-40.
5. Log from the vial. Each dose in the dose log that names this vial takes one off the count. At three left, a refill reminder fires.

The maths, and where it goes wrong

The arithmetic is short. Concentration is the peptide divided by the water. Volume to draw is the dose divided by the concentration. On a U-100 insulin syringe, 100 units is 1 mL, so units are the volume multiplied by 100. On a U-40 syringe, 40 units is 1 mL.

Three things make this go wrong in practice. The first is mixing up milligrams, millilitres and units, which are three different quantities. The second is the syringe. A U-40 syringe has fewer marks per millilitre than a U-100 one. If you calculate units for one scale and draw on the other, the dose is off by a factor of 2.5, which is why the makers of U-40 veterinary insulin tell users never to use it with a U-100 syringe. The third is very small volumes: on a 100-unit syringe, a draw of 2 or 3 units is hard to read accurately, and adding more water to the vial is usually the fix because it lowers the concentration and raises the volume for the same dose. The small dose explainer and the syringe units guide go through it in detail.

This is not hypothetical. In July 2024 the FDA said it had received reports of dosing errors with compounded injectable semaglutide in multiple-dose vials. Patients had taken 5 to 20 times the intended dose, often after confusion between milligrams, millilitres and units, and some providers had miscalculated when converting milligrams to units, producing doses 5 to 10 times the intended amount. One example in the alert: patients told to draw 5 units (0.05 mL) drew 50 units instead. The FDA asked providers and compounders to supply the right syringe size and to counsel patients on measuring the dose. A calculator does not replace that conversation. It does remove the arithmetic from the moment you are holding a syringe.

Why doses left and expiry matter

Bacteriostatic water is sterile water with benzyl alcohol added as a preservative, at 0.9 percent in the common product, which is what allows a vial to be punctured more than once. It does not make a vial last forever. In clinical settings The Joint Commission's rule for opened multi-dose vials is to discard them 28 days after opening unless the manufacturer specifies otherwise. Reconstituted peptides also lose stability over time in the fridge, and the right window varies by compound. The storage guide covers what is known. An expiry date on each vial, set when you mix it, takes the guesswork out of whether the one at the back of the fridge is still in date.

The dose count does a simpler job: it stops you discovering on the morning of a dose that the vial is empty. The refill reminder at three doses is set so there is time to sort the next vial before the gap.

Who it is for

• Anyone drawing from a multi-dose vial rather than using a prefilled pen, including people on a compounded GLP-1 who have been given a vial and syringes.
• People running more than one reconstituted compound at once, each with its own concentration and expiry.
• Anyone who has ever had to redo the maths because a new vial came in a different size.

How it connects to the rest of the app

Each vial is linked to doses in the dose log, and the protocol planner knows how many doses the current protocol uses each week. Compound pages in the library carry reconstitution details. The site on each dose goes to the rotation map. The same calculation is on the site, without the app, on the calculators page and in the per-compound guides such as semaglutide reconstitution and tirzepatide reconstitution.

When you mix your next vial, enter it in the app before the first draw: the size, the water, the date. Every dose after that counts down on its own. The app is on the download page.