Peptide pen plunger not working? Check the cartridge first

This page covers the three pen problems we hear about most: a plunger that won't reach the stopper, a cartridge holder that won't thread on, and a plunger that feels stuck. It's hardware only. Work through the checks in order; the cartridge format comes last because the first two checks take seconds.

First, two words: plunger and stopper

On this page the plunger is the pen's drive rod, the part that moves forward to push. The stopper is the rubber plug sealing the back of the glass cartridge. Engineering standards call the rubber part the "plunger" and the rod the "plunger rod", and product listings use both words, so when a listing quotes a plunger height in mm, it means the rubber stopper.

Problem 1: the plunger won't reach the stopper

You load a fresh cartridge, put the holder back on, and there's still a gap between the drive rod and the rubber stopper. Check these three things, in this order.

1. Is the drive rod reset? A fresh cartridge needs the rod back at its start position. In the Gansulin pen manual the loading steps are: open the holder, put in a cartridge, push the internal drive rod all the way back so it sits flush, then twist the loaded holder back onto the body until it seats snugly. For a V1, V2 or V3, follow the loading instructions for that pen.
2. Is the holder fully seated? A holder that is only part of the way on holds the cartridge further from the rod. Twist it on until it stops.
3. Still a gap? Check the cartridge format. If a short-stopper (about 8 mm) cartridge goes into a pen built for the long (about 11 mm) stopper, the stopper sits deeper in the glass, so the rod stops short of it. Nothing is broken. The pen and the cartridge are different formats.

The fix: use a cartridge with the stopper height your pen is built for. For all four of our pens that is the 11 mm long plunger: our clear 3 ml glass cartridges or amber 3 ml glass cartridges.

Problem 2: the cartridge holder won't thread on

This is the mirror image of Problem 1. If a long-stopper (about 11 mm) cartridge goes into a pen built for the short (about 8 mm) stopper, the rod meets the stopper before the holder is fully on, so the holder stops partway or feels stiff. Makers of short-stopper pens say a long-stopper cartridge should not be used in them.

• Reset the drive rod first, as in Problem 1, before you screw the holder on.
• Check the format. If you have one of our pens and a long-plunger cartridge, the format is not the cause, because our pens are built for the long plunger. If you have a pen from somewhere else, check which stopper height that pen is built for.

Problem 3: the plunger feels stuck or stiff

• Wrong format. A long-stopper cartridge in a short-stopper pen is the stiff-assembly case from Problem 2.
• A cartridge that sat compressed. A cartridge stored for months with its stopper driven hard against the base can drag on first use. If a pen is going to sit unused for a long stretch, store it with the cartridge removed so the seal isn't held under compression.
• Everything checks out and it still won't move? We don't sell drive rods or other internal pen parts separately, so contact us with your order number and a photo of the pen with the cartridge loaded.

How to tell which cartridge format a pen takes

The glass won't tell you. The international standard for pen cartridges (ISO 13926-1) fixes the glass, an 11.6 mm outside diameter for a 3 ml cartridge, which is why every 3 ml cartridge looks the same and fits the same holder. The companion standard for the rubber stopper (ISO 13926-2) leaves its height to be agreed between the manufacturer and the user. In practice that means two heights on the market, about 8 mm and about 11 mm.

• The pen: its listing or manual should say short or long stopper, or give the height in mm. All four of our pens, the Gansulin, V1, V2 and V3 peptide pens, are built for the 11 mm long plunger.
• The cartridge: check the box or listing for the stopper height. If it doesn't say, ask the seller before you load it.
• Look through the glass: our 11 mm long plunger is about as tall as the cartridge barrel is wide. A short stopper is clearly shorter than the barrel is wide.

To compare a cartridge you already have against ours, use the cartridge fit checker. All our cartridges are on the 3 ml cartridges page.

Common questions

Why is my peptide pen plunger not working?

Usually the cartridge is the wrong format for the pen. 3 ml cartridges come with a rubber stopper about 8 mm or about 11 mm tall, and a pen is built for one height. A short stopper in a pen built for the long one sits deeper in the glass, so the plunger stops short of it. Reset the drive rod and seat the holder fully first; if there's still a gap, check the stopper height.

Why is my peptide pen not working properly?

Go through three checks: the drive rod is reset for a fresh cartridge, the cartridge holder is screwed fully on, and the cartridge's stopper height matches the pen. Our Gansulin, V1, V2 and V3 pens are built for the 11 mm long plunger cartridge we sell.

Peptide pen plunger stuck: what should I check?

Check that the cartridge isn't a long-stopper cartridge in a pen built for the short stopper, which makes the assembly stiff. A cartridge stored for months with its stopper pressed hard against the base can also drag on first use. If the format is right and it still won't move, contact the seller.

V2 peptide pen not working: is it the pen?

Usually not. The V2 is a manual draw-and-push pen built for the 11 mm long plunger cartridge. The same three checks apply: reset the drive rod, seat the holder fully, and confirm the cartridge has the 11 mm long plunger.

Why won't the cartridge holder screw on to my pen?

The most common cause is a long-stopper cartridge in a pen built for the short stopper: the rod meets the stopper before the holder is fully on. Reset the drive rod first, then check which stopper height your pen is built for.

Which cartridge fits my peptide pen?

A 3 ml glass cartridge with the stopper height your pen is built for. All four of our pens take the standard 3 ml glass cartridge with the 11 mm long plunger, which is the cartridge we sell in clear and amber.

Do all 3 ml pen cartridges fit every pen?

The glass does: a 3 ml cartridge has an 11.6 mm outside diameter under ISO 13926-1, so it fits the holder. The rubber stopper height is left to the maker, about 8 mm or about 11 mm, and that decides whether the plunger meets the stopper correctly. The two look identical from the outside.

How do I load a cartridge in a peptide pen?

The Gansulin pen manual's loading steps are: open the cartridge holder, put in a cartridge, push the internal drive rod all the way back so it sits flush, then twist the loaded holder back onto the body until it seats snugly. For a V1, V2 or V3, follow the loading instructions for that pen.

For research use only. Not for human or veterinary use.