How a cartridge pen works: loading, priming and dialing.
A pen is a syringe with the measuring built in: a cartridge of solution, a screw that pushes the cartridge's plunger a fixed distance per click, and a dial that counts the clicks. Priming clears air from the needle and proves the mechanism actually moves liquid.
There are two very different kinds of pen, and the difference decides which instructions you follow. A manufacturer's prefilled pen is a drug and a device approved together. A reusable pen is a device you load with cartridges, and whether its markings mean anything depends on whether the pen and the cartridge were made for each other.
This tutorial explains the mechanics both kinds share. It is background for reading your pen's own Instructions for Use, not a replacement for them.
Reviewed October 14, 2026
This page never gives a click count for any amount, and it does not cover injection technique. For a prefilled pen, the product's Instructions for Use are the manual.
What you need.
- The pen: prefilled, or reusable plus a cartridge made for it
- The pen's Instructions for Use
- A new pen needle that fits your pen
- Alcohol swabs
- A sharps container
Step by step.
Work out which kind of pen you have
Prefilled pens are made by the drug's manufacturer, arrive with the medicine sealed inside, and are approved as one product. For these, the product's Instructions for Use are the only manual that matters. They are public: the Ozempic label and Instructions for Use, for example, are on DailyMed.
Reusable pens are a durable device that takes replaceable cartridges. Some approved products work this way: the Genotropin label describes its pen as a reusable multi-dose device holding a two-chamber cartridge, and points users to the directions supplied with each device. Generic reusable pens are also sold empty, with no drug and no approval linking them to any cartridge. With those, nobody has checked that the pen, the cartridge and the solution work together.
- Needle. New for every use. Priming is complete when a drop appears at its tip.
- Cartridge seal. The rubber septum the needle passes through. Swab it and let it dry.
- Cartridge. Holds the solution. Check it is the right product, in date and clear.
- Rubber plunger. Pushed forward a fixed distance for every click.
- Screw (plunger rod). Turns the dial's rotation into forward travel.
- Dose window. Shows the pen's count of steps, in the units the pen was designed around.
- Dose knob. Each click is one fixed step of volume, not a fixed amount of drug.
- Push button. Delivers what was dialed. Turning the dial back down delivers nothing.
The parts most pens share. Priming is complete when a drop appears at the needle tip. Load the cartridge (reusable pens only)
Prefilled pens skip this step. For a reusable pen, wash your hands, unscrew the cartridge holder from the pen body, and check the cartridge before it goes in: right product, in date, glass intact, rubber seal clean, solution clear with no particles. Many reusable pens need the plunger rod wound back before a fresh cartridge will seat, and the pen's manual says how. Seat the cartridge, screw the holder back on until it stops, and never force a cartridge that does not fit; one made for a different pen may seat badly and deliver a different volume than the markings assume.
Do not use a pen cartridge as a vial. ISMP has documented staff drawing doses out of pen cartridges with a syringe, a practice it notes can introduce air into the cartridge and lead to under-delivery afterward.
Attach a new needle every time
Swab the rubber seal at the front of the cartridge holder with alcohol and let it dry. Peel the paper tab from a new pen needle, screw or push it on straight, then remove the outer cap (keep it for later) and the inner cap. The Ozempic Instructions for Use put the rule plainly: always use a new needle for each injection. A used needle is no longer sterile, can be blocked, and is a channel for air and contamination into the cartridge.
One pen, one person, always. CDC's injection safety guidance says a pen is for a single person even when the needle is changed, because blood may be present in the pen after use, and the Ozempic label states that pens must never be shared between patients, even if the needle is changed.
Prime it: do a safety shot until a drop appears
Priming, also called a safety shot or flow check, pushes a small set amount through the needle into the air with the pen pointing up. It does two jobs: it clears air out of the cartridge and needle, and it proves the mechanism is actually moving liquid before anything is measured.
When and how much to prime is set by the pen, which is why its instructions decide. The Ozempic Instructions for Use, for example, call for a flow check before the first injection with each new pen only, using a dedicated flow check symbol on the dial, and say a drop should appear at the needle tip. Other pens are primed differently.
The method is the same idea everywhere: hold the pen needle up, tap the cartridge holder so any bubbles rise, press the button fully, and watch the tip. You want to see a drop. If none appears, follow the pen's troubleshooting steps, which usually means repeating the prime and then trying a new needle. A pen that still will not produce a drop should not be used, because it cannot be trusted to deliver anything.
Dial the setting and understand the clicks
Turning the dose knob turns a screw inside the pen. Each step you feel or hear, a click, advances that screw a fixed distance, and each fixed distance moves the cartridge's plunger by a fixed volume. The number in the dose window is the pen's count of those steps, printed in whatever units the pen was designed around.
That translation is the catch. A click is a volume, and what that volume contains depends on the concentration of the cartridge in the pen. On a prefilled pen the window shows the labeled amount directly, because pen and solution were designed together. On a reusable pen loaded with a cartridge it was not designed for, the numbers in the window may not correspond to anything printed on that cartridge.
Pens also differ in step size. Among insulin pens, for example, standard pens dial in whole-unit steps, some dial in half units, and some only in two-unit steps. The GLP-1 pen clicks page explains the mechanics of the common GLP-1 pens and derives a per-click figure only from totals you enter for your own pen. This tutorial gives no click count for any amount, and dialing a prefilled pen to settings between its labeled ones is off-label.
Remove the needle and store the pen
After use, take the needle off straight away using its outer cap, put it in the sharps container, and replace the pen cap. The Ozempic instructions say to always remove the needle, and its label says to store the pen without a needle attached. A needle left on is an open channel between the cartridge and the room.
Store the pen exactly as its label says. In-use windows and temperatures differ between products and even between presentations of the same drug; the storage and shelf life reference lists them with a link to each label. Note the date you first used the pen or loaded the cartridge somewhere you will see it.
This tutorial covers the device up to a dialed, primed pen. It does not cover injecting. Administration technique comes from your prescriber, pharmacist, or the product's Instructions for Use.
Common mistakes.
Following general advice over the pen's own instructions
Priming rules, needle types and storage differ by pen. The Instructions for Use for your pen win.
Reusing pen needles
A new needle every time. Used needles dull, block, and let air and contamination into the cartridge.
Skipping the prime on a new pen or cartridge
Without it, the first measurement can include air instead of solution.
Storing the pen with the needle on
Labels say to remove it. A fitted needle lets solution out and air in.
Sharing a pen, even with a new needle
CDC and product labels both say no. Blood can be present in a pen after use.
Turning the dial back to zero instead of pressing the button
ISMP lists this among pen errors: the dial resets and nothing is delivered.
Drawing from a pen cartridge with a syringe
ISMP notes this can let air into the cartridge and cause later under-delivery.
Treating a click as the same amount on every pen
A click is a fixed volume for that pen. What it contains depends on the cartridge.
FAQ.
How many clicks is a given amount?
It depends on the pen and on the concentration of the cartridge in it, so there is no universal number, and we do not publish click counts for any amount. For a prefilled pen, use the settings printed in its Instructions for Use. The GLP-1 pen clicks page explains the mechanics and only calculates a per-click figure from totals you enter for your own pen.
Why prime if the needle is brand new?
A new needle is full of air, and a new cartridge can hold an air gap in front of the plunger. Priming pushes that air out and proves solution is flowing. Without it, part of the first measurement can be air.
Does priming waste solution?
A small amount, by design, which is why each product's instructions say exactly when to prime. Priming more often than instructed uses solution up faster than planned.
Can a reusable pen take any cartridge?
No. A pen is engineered around a specific cartridge's dimensions and, in approved systems, a specific solution. A cartridge from another system may not seat correctly, and its clicks no longer match the volume the pen's markings assume.
Is it safe to share a pen if I change the needle?
No. CDC guidance and product labels both say a pen is for one person only, even when the needle is changed, because blood may be present in the pen after use.
Sources.
- [1]Ozempic (semaglutide) label and Instructions for Use · DailyMed, US National Library of Medicine
- [2]Genotropin (somatropin) label · DailyMed, US National Library of Medicine
- [3]Considerations for Blood Glucose Monitoring and Insulin Administration · CDC, Injection Safety
- [4]ISMP Guidelines for Optimizing Safe Subcutaneous Insulin Use in Adults · Institute for Safe Medication Practices, 2017
- [5]McKee AM et al.: The Nuances Surrounding Insulin Prescribing · Clinical Diabetes, 2023 (PMC10338272)
Cite this page
PepCue. “How a cartridge pen works: loading, priming and dialing.” PepCue, reviewed October 14, 2026. https://www.pepcue.app/tutorials/peptide-pen.
All four walkthroughs are on the tutorials hub.