Compounded Semaglutide Vial vs Pen — What's the Difference?

The short answer

Ozempic and Wegovy are pre-filled, pre-dosed brand pens. You dial the dose on the pen and inject — no syringe, no vial, no calculation needed. The calculator on this site is not designed for those devices and will give incorrect results if you try to use it with them.

Compounded semaglutide is the same active molecule, manufactured by a compounding pharmacy and supplied in a multi-dose glass vial. You draw from the vial yourself using an insulin syringe. Because the concentration printed on your vial (in mg/mL) determines how many mL to draw for each dose, a calculator is needed. That is exactly what the semaglutide calculator does.

Side-by-side comparison

How a compounded vial works

A compounded semaglutide vial is a small glass container, typically 3–10 mL, sealed with a rubber stopper. The medication is dissolved in liquid at a specific concentration — most commonly 2.5 mg/mL, 5 mg/mL, or 10 mg/mL. Your prescriber tells you the dose in mg; your concentration label tells you how strong the solution is. You divide dose by concentration to get the volume to draw, then draw that volume into an insulin syringe and inject subcutaneously.

This is why the calculator asks for two inputs: your dose (mg) and your vial concentration (mg/mL). Enter both and it gives you the exact volume in mL and the equivalent mark on a U-100 insulin syringe.

How a brand pen works

Ozempic and Wegovy are pre-filled injection pens. Each pen contains a fixed amount of semaglutide dissolved in solution. You attach a new needle, dial the dose in mg using the dose selector wheel, prime the pen, and inject. The pen delivers exactly the dialled amount — there is no calculation involved and no syringe to fill.

Because the pen's internal mechanism controls the volume delivered, entering a pen dose into a draw-volume calculator will give a meaningless result. The calculator assumes you are filling a syringe from a vial — a completely different process.

How to tell which one you have

• It's a vial if: it's a small glass bottle with a rubber cap on top, you draw from it with a separate insulin syringe, and the label shows a concentration in mg/mL (e.g., "Semaglutide 5 mg/mL").
• It's a pen if: it looks like a thick marker or epi-pen with a dose window, you attach a needle tip, and there is a dial or button to set the dose. The label will say "Ozempic", "Wegovy", or show a pen-device diagram.

If you received it from a compounding pharmacy or a telehealth prescriber and it came in a glass vial — it's almost certainly a compounded vial. Use the semaglutide calculator.

If you have Ozempic or Wegovy from a retail pharmacy — you have a brand pen. Do not use the calculator. Follow the pen instructions.

Why compounded vials require more attention

Because concentration varies between compounding pharmacies, two people prescribed the same dose may draw different volumes if their vials have different concentrations. For example, a 0.5 mg dose requires 0.20 mL from a 2.5 mg/mL vial, but only 0.10 mL from a 5 mg/mL vial. Using the wrong concentration in your calculation results in under- or over-dosing. Always check your label.

Compounded vials also require correct storage, handling technique, and clean injection practice — the vial is accessed multiple times over several weeks. See the GLP-1 storage guide for specific guidance.

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