Vial Transition Dosage Calculator

Switching vial concentrations safely

Pharmacies sometimes dispense a new vial at a different concentration — often more concentrated, so the volume per dose is smaller. Your dose in milligrams doesn't change, but the units you draw do. This is the single most dangerous moment in compounded GLP-1 dosing, because nothing about the injection feels different. Same syringe, same routine, same number you've drawn for weeks — and potentially double the drug.

The math, and why it inverts

Units on a U-100 syringe measure volume, not drug. 100 units = 1 mL, so 1 unit = 0.01 mL, always, regardless of what's in the vial. The milligrams that ride along in that volume are entirely determined by concentration:

units = (dose in mg ÷ concentration in mg/mL) × 100

Concentration sits in the denominator, which is why the relationship runs backwards from intuition: a stronger vial needs fewer units. Doubling the concentration halves the units. Readers who expect "stronger medicine, bigger number" get this exactly wrong, and the error is always in the overdose direction.

Worked example: the same 5 mg, three ways

Say your prescribed dose is 5 mg and it doesn't change. Here is what the syringe should read on three different vials:

Carry the old 50 units onto that 20 mg/mL vial and you have just injected 10 mg — double your prescription. That is a bigger jump than any single step on the official titration ladder, taken without the four-week adjustment period those steps are designed around.

The label trap: mg per 0.5 mL is not mg per mL

Most mis-set concentrations start with a misread label. A vial marked "5 mg/0.5 mL" is 10 mg/mL — halving the volume doubles the concentration. Anyone who types "5" into the concentration field from that label will be told to draw 100 units instead of 50, and inject twice the intended dose.

To normalise any label to mg/mL, divide the milligrams by the millilitres: 5 mg ÷ 0.5 mL = 10 mg/mL. If your vial states a total strength and a total volume — say "60 mg/2.4 mL" — the same division gives 25 mg/mL. The calculator accepts either form so you don't have to do this in your head, but you still have to read the label correctly to enter it.

Where this tool does not apply

Branded pens and single-dose vials never need this math. Zepbound, Mounjaro, Ozempic and Wegovy pens deliver fixed, pre-set doses — there is nothing to convert and no syringe to draw. Zepbound single-dose vials are supplied as a fixed milligram amount in 0.5 mL, and the multi-dose vial is designed around a 0.6 mL dose volume rather than 0.5 mL. Unit conversion is a compounded-vial problem, which is also the setting with the least standardisation and the most room for a labelling surprise.

Two habits are worth more than any calculator here. Re-read the concentration on every new vial rather than assuming the refill matches the last one, and have your pharmacist confirm your unit count in writing the first time a strength changes. A dose you have drawn a hundred times is exactly the one you will draw on autopilot.