How to read the concentration on a compounded vial

Brand pens hide the concentration from you: the pen delivers a fixed dose and the arithmetic was done at the factory. A compounded vial does not. The label carries a concentration, and the concentration is the one number every dose calculation depends on. Here is how to read it.

The four ways it is written

As a rate: "10 mg/mL". Already in the form you need. Ten milligrams of drug in every millilitre of liquid.

As totals: "5 mg / 2 mL" or "2.5 mg in 1 mL". Divide the mass by the volume. 5 mg over 2 mL is 2.5 mg/mL. The volume here is the fill volume, and it may not equal the vial's nominal size (a 3 mL vial often holds 2 mL of product).

As a total mass only: "10 mg vial". Common for lyophilised (powder) products that you reconstitute. There is no concentration until diluent is added; the concentration becomes total mg divided by the mL of diluent you add. The reconstitution guide covers this.

As a per-unit instruction: "25 units = 2.5 mg". The pharmacy has done the conversion for one concentration. Back it out to check: 25 units is 0.25 mL, and 2.5 mg over 0.25 mL is 10 mg/mL. If the label's own rate line does not agree with that, call the pharmacy before using the vial.

Things that look like the concentration but are not

• The vial size. "3 mL vial" is glass capacity, not fill volume, and says nothing about mg.
• The total drug per vial on its own. 10 mg in a vial is a different concentration at 1 mL than at 2 mL.
• A dose instruction. "Inject 0.25 mL weekly" is what to draw, not what is in it.
• A strength in mg per syringe on pre-filled compounded syringes. That is a dose, not a rate; useful, but do not divide anything by it.

Multi-ingredient vials

Some compounded products combine a GLP-1 with a second ingredient, most often cyanocobalamin (vitamin B12). The label then carries two concentrations, for example "2.5 mg / 0.5 mg per mL". The first number in these listings is usually the GLP-1, but the order is a pharmacy choice, not a rule. Find the ingredient name next to each number and use only the GLP-1 concentration in the converter.

When the label and the prescription disagree

If the pharmacy label says "draw 40 units" and your own arithmetic from the concentration and the prescribed mg gives 20 units, do not split the difference and do not assume either is right. One of three things has happened: the concentration changed at this fill and the instruction was not updated, the prescription changed and the label was not, or you have misread one of them. Call the pharmacy with the vial in your hand and read them the lot number.

A short label-reading routine

1. Find the GLP-1 ingredient name and the number beside it.
2. Write the concentration as mg/mL, dividing totals if needed.
3. Compare with the previous vial's concentration. If it changed, recompute the volume before the first dose.
4. Compute volume: prescribed mg divided by mg/mL. Check it in the converter.
5. Confirm the pharmacy's own units line agrees. If not, call.

Compounded semaglutide and tirzepatide are not FDA approved, and the label is the pharmacy's statement of contents rather than an FDA-reviewed one. The concentration on it is the best information you have, which is a reason to read it carefully, not a reason to trust it blindly.