Running short on a pen is rarely about the pen. It is about timing. A reorder date comes from two numbers: when the pen will run out, and how long it takes to get the next one. Work out both, and you can put a date on the calendar instead of noticing too late.
Find the run-out date
Start with what the pen holds when new and what each use takes. Both come from your pen's label, your pharmacy or your prescriber's instructions. Then count how many uses you have in a typical week.
- Uses per pen = total in the pen ÷ amount per use.
- Weeks of use = uses left ÷ uses per week.
- Run-out date = the date you opened it + weeks of use.
Some pens are set by clicks or dial steps rather than by volume. Use whatever unit your pen's instructions use, as long as the total and the amount per use are in the same unit.
Work out your lead time
Lead time is everything between deciding to reorder and having the new pen in hand. It is usually longer than it feels. Add up each step.
- Time to request a refill or a new prescription.
- Pharmacy processing or approval time.
- Shipping, or the trip to pick it up.
- Weekends, holidays and any travel of your own.
Base the total on your slowest recent refill, not your fastest. Then add a buffer of a few days for the step that sometimes goes wrong.
Set the reorder date
Now subtract. Reorder date = run-out date − lead time − buffer. That is the last comfortable day to start the process.
| Daily pen total | 3.0 mL |
|---|---|
| Amount per use | 0.25 mL |
| Uses per pen | 3.0 ÷ 0.25 = 12 |
| Uses per week | 2 |
| Weeks of use | 12 ÷ 2 = 6 weeks (42 days) |
| Opened | March 1 |
| Run-out date | About April 12 |
| Lead time | 7 days |
| Buffer | 3 days |
| Reorder date | April 12 − 10 days = April 2 |
These are example numbers. Use your own pen's label and follow your prescriber's instructions.
Put the reorder date where you will see it: a calendar entry, a reminder or a note on the pen box. A date you have to look up is easy to miss.
Count what is already on the shelf
If you keep an unopened spare pen or cartridge, your real run-out date is later. Add the uses in your unopened stock to the uses left in the open pen, then redo the math with the new total.
Keep that count current. When you open a spare, it moves from stock to in use, and your stock goes down by one. Your next reorder date should come from the last item you have, not the first.
Check the expiry date on every spare. A spare that expires before you reach it does not extend your run-out date.
Doing it in an app or on paper
A calendar and a pencil are enough. Write the run-out date, count back the lead time and buffer, and mark the reorder date. Redo it each time the amount per use or your schedule changes, because both dates move.
In an app, the dates move for you. PeptidePal keeps pens and cartridges on their own tab, works out the run-out date from what you log, and can send low-volume warnings and expiry alerts. It still only knows what you enter, so log each use as it happens.
Either way, a reorder date is a plan, not a guarantee. Leave room for the slowest step.
Questions
What lead time should I use?
Your slowest recent refill, plus a few days. If you have not reordered before, ask your pharmacy how long it usually takes.
What if my schedule changes?
Redo the math from the uses left today. A change in amount per use or uses per week moves both the run-out date and the reorder date.
Does a spare pen change my reorder date?
Yes, if it is unopened and in date. Add its uses to what is left, then subtract lead time from the new run-out date.
Written and reviewed for accuracy by the PeptidePal team at SignalWorks Labs. This guide is general record-keeping help, not medical advice. Follow your prescriber's instructions and the product label. More in the guides or on the support page.