Fundamentals
Pens versus vials
Last reviewed · Written by the PepCycle team
There are three formats, not two. A manufacturer pen delivers a fixed amount set by the device. A manufacturer vial contains a finished solution at a stated concentration. A lyophilised vial contains powder you dissolve yourself. Only the third involves reconstitution maths — and confusing the second with the third is the most common tracking error in GLP-1 use.
The three formats
| Manufacturer pen | Manufacturer vial | Lyophilised vial | |
|---|---|---|---|
| Contents | Finished solution | Finished solution | Dry powder |
| Concentration set by | The manufacturer | The manufacturer | You, when you add diluent |
| You measure | Nothing — the device does | A volume, with a syringe | A volume, with a syringe |
| Reconstitution? | No | No | Yes |
| Calculator involved? | Never | Only unit ↔ volume | Yes, full three-step |
| Examples | Ozempic, Wegovy | Some Mounjaro presentations, compounded vials | BPC-157, TB-500 |
The word "vial" doing double duty across columns two and three is the root of most of the confusion. They are physically similar objects with completely different logging requirements.
Manufacturer pens
A pen is a metering device. Whether it delivers a fixed amount (single-dose) or a selected one (dial pen), the amount is defined by the device and its instructions. Nothing is derived, and nothing should be.
- Record the product, strength and lot as printed.
- Record the dial setting for multi-dose pens, in the units the instructions use.
- Record the first-use date, because pens have an in-use window separate from the printed expiry.
- Never convert a pen dose to millilitres or syringe units. The number has no use and inviting the conversion invites error.
Manufacturer and compounded vials
A manufacturer or pharmacy vial contains a finished solution at a concentration determined and verified by whoever filled it. You are not creating a concentration; you are drawing a volume from a known one.
- Record the stated concentration exactly — it is the only number that lets volume convert to amount.
- Record the syringe type, since the barrel calibration determines what you read. See reading an insulin syringe.
- Record the beyond-use date for compounded product, and the first-puncture date for any multi-dose vial.
- Never assume a new vial matches the last one's concentration. Start a new vial record every time — see compounded GLP-1 medications.
Lyophilised vials
This is the only format where a reconstitution calculator has a role, because it is the only one where you determine the concentration. The full three-step arithmetic applies, and the diluent volume becomes a permanent part of the record.
See reconstitution explained for the arithmetic, and use the reconstitution calculator rather than doing it in your head.
Why the distinction matters for a tracker
Most tracking apps pick one format and treat everything else as an awkward special case. Peptide-first apps push GLP-1 pens through a reconstitution flow that makes no sense. GLP-1-first apps cannot represent a vial you dissolved yourself at all.
PepCycle makes format a property of the compound, so a pen, a manufacturer vial and a lyophilised vial each present the fields that actually apply — and only those. See how it compares to trackers built around a single format.
Frequently asked questions
Do I need to reconstitute a Mounjaro or Zepbound vial?
No. Manufacturer-filled vials contain a finished solution at a fixed, verified concentration. Reconstitution applies only to lyophilised powder that you dissolve yourself.
Can I convert my pen dose into syringe units?
You should not. A pen's amount is defined by the device and its instructions, and converting it to a volume serves no purpose while introducing a chance of error. Record it exactly as the instructions express it.
What is the difference between a compounded vial and a research peptide vial?
A compounded vial contains a finished solution prepared by a pharmacy at a stated concentration; a research peptide vial contains dry powder you dissolve yourself. Only the second involves reconstitution arithmetic — but both require you to draw a volume with a syringe.
Why does my tracker keep asking about bacteriostatic water for my pen?
Because it models every compound as a lyophilised vial. That is a sign the app was built for one format and is treating yours as a special case, which is exactly where logging errors come from.
Related pages
Log it once, keep it forever
PepCycle turns the numbers on this page into a running record: every dose, vial, site and side effect in one place — stored on your iPhone and backed up to your own iCloud.
Free to download · No account required · Price shown before you enter anything personal