Every pre-filled peptide pen looks identical from the outside. The detail that decides fit sits inside it, in specs most listings never print: cartridge length and stopper depth.
Our pens take standard 3 ml (300-unit) glass cartridges with the 11 mm long plunger (stopper), the cartridge we sell. 3 ml cartridges are also made with a shorter, about 8 mm plunger, and a pen is built for one height, so check that a cartridge from another source has the 11 mm long plunger before loading it; the glass looks identical from the outside. Needle fit is universal: our pens take any 28G–33G screw-on pen needle.
What it is
Key takeaways
- The dial counts clicks, not microliters; its value depends on cartridge bore and fill volume.
- Cartridge fit is strict: Our pens take standard 3 ml (300-unit) glass cartridges with the 11 mm long plunger (stopper), the cartridge we sell. 3 ml cartridges are also made with a shorter, about 8 mm plunger, and a pen is built for one height, so check that a cartridge from another source has the 11 mm long plunger before loading it; the glass looks identical from the outside.
- Pen needles are standard: any universal 28G-33G screw-on pen needle threads onto our pens.
- A concave plunger face traps roughly 0.10-0.15 ml of sample; flat-face cartridges cut that below 0.08 ml.
- Never run a second peptide through a used cartridge; residual dead space cross-contaminates the next reconstitution.
In this article
A pre-filled peptide pen is a reusable click-dial device, usually metal-bodied, that holds a 3 ml glass cartridge of reconstituted peptide. Rather than drawing from a vial for every aliquot, the dial advances the cartridge plunger a fixed distance, dispensing the same nominal volume each time.
Two words carry the weight. Nominal, because the dial markings are arbitrary ratchet counters, not microliters — the real volume depends on cartridge bore and fill level. Cartridge, because the pen is only a holder: accuracy, sterility window and sample waste are set by the glass, the stopper and the plunger face, not the housing.
Which cartridges and needles fit
Two interfaces decide that, and neither is negotiable.
Compare
| Interface | What fits our pens | What it changes |
|---|---|---|
| Cartridge | Standard 3 ml glass, 11 mm long plunger | a plunger (stopper) height that differs from ours; the glass looks identical |
| Needle | Universal 28G-33G screw-on pen needles | Set by the pen-needle thread, not the cartridge |
| Dial | Arbitrary ratchet units | Not microliters; convert before trusting a click count |
On third-party hardware we stay quiet: we have not tested other makers' pens, and whether one of our cartridges seats in a device we have never handled is not something we'll guess at. Looking like it fits is not the same as seating and advancing cleanly.
Key point: Dial numbers are arbitrary ratchet counters, not microliter volumes — cartridge bore variance and dead space decide what actually leaves the needle.
All of our reusable peptide pens (V1, V2, V3 and Gansulin) are compared on one page.
Metal vs plastic: the housing material that actually matters
Key numbers
Two housing types dominate: reusable metal bodies that accept a replaceable 3 ml glass cartridge, and disposable plastic auto-injectors pre-filled at a factory. For lab work, the metal reusable body is the more controllable hardware.
Protection and ratchet mechanics
A metal housing—anodized aluminum or a zinc-alloy core—shields the glass cartridge from light. Peptides degrade faster under UV and fluorescent light; a metal barrel blocks that, while a plastic body does not.
The click-dial on a quality metal pen is a precision ratchet. Each click advances the plunger a fixed linear distance, and the volume displaced depends on the cartridge's inner diameter.
Plastic gears and repeatability
Disposable plastic pens use a spring-loaded plunger and molded plastic gear train. Metering becomes less repeatable after roughly 10 to 15 dispenses as plastic gears wear microscopically. Dispense-to-dispense variation can drift by 3 to 5 percent—enough to blur a concentration-response curve.
Cartridge fit
Our pens take standard 3 ml (300-unit) glass cartridges with the 11 mm long plunger (stopper), the cartridge we sell. 3 ml cartridges are also made with a shorter, about 8 mm plunger, and a pen is built for one height, so check that a cartridge from another source has the 11 mm long plunger before loading it; the glass looks identical from the outside. They also take universal 28G–33G screw-on pen needles. We have not tested other manufacturers' pens, so we cannot confirm fit with third-party devices.
The glass cartridge numbers nobody talks about
A 3 ml glass cartridge is a set of dimensions before it is a volume. Four of them decide whether it seats, how far the dial actually moves liquid, and how much never comes out.
Which cartridges fit our pens
Two numbers decide it, and neither is printed on a product page.
Nominal , because the dial markings are arbitrary ratchet counters, not microliters — the real volume depends on cartridge bore and fill level.
- Plunger (stopper): 11 mm long plunger, the height our pens are built for. Our pens take standard 3 ml (300-unit) glass cartridges with the 11 mm long plunger (stopper), the cartridge we sell. 3 ml cartridges are also made with a shorter, about 8 mm plunger, and a pen is built for one height, so check that a cartridge from another source has the 11 mm long plunger before loading it; the glass looks identical from the outside. No competitor discloses this.
- Bore diameter: 8.65 mm, plus or minus 0.05 mm. The DIN/ISO nominal for a 3 ml cartridge, and the dimension the click geometry is built around.
Do the cartridges work with other pens?
We can't tell you, and we won't guess. We have not tested other manufacturers' pen bodies, so we make no fit claims about hardware we don't build. The check is mechanical, not brand-based: Our pens take standard 3 ml (300-unit) glass cartridges with the 11 mm long plunger (stopper), the cartridge we sell. 3 ml cartridges are also made with a shorter, about 8 mm plunger, and a pen is built for one height, so check that a cartridge from another source has the 11 mm long plunger before loading it; the glass looks identical from the outside.
Why the bore number matters
Bore is held to 8.65 mm, plus or minus 0.05 mm. A cartridge at the wide end of that tolerance dispenses roughly 0.6 percent more liquid per click than one at the narrow end, and the error compounds across a full dial. If a pen's click ratio is set for one specific drug concentration, the dial number is a relative reference, not a microlitre reading.
The dead volume you pay for
When the plunger bottoms out, liquid stays trapped in the neck, around the septum and inside the crimp cap: 0.08–0.12 ml on a standard 3 ml cartridge, and up to 0.15 ml behind a domed plunger face. Reconstituting 10 mg into 2 ml leaves roughly 0.5 mg in that residue — volume the dial cannot reach.
Compare
Bacteriostatic water and the pre-fill gamble
Bacteriostatic water is a preservative, not a stabilizer. The 0.9% benzyl alcohol in it holds bacterial growth down; it does nothing to stop hydrolysis, and it can itself interact with peptides that carry exposed hydrophobic residues.
Does benzyl alcohol affect peptide stability in solution?
Yes, but sequence-specifically rather than universally. In a pre-filled cartridge, hydrolysis is usually the dominant loss route, with benzyl alcohol a secondary contributor. Neither is visible from the outside: a clear cartridge tells you nothing about what is left in it.
Pre-fill storage windows
- 4 °C: most sequences hold for 7–14 days.
- Room temperature: fragile sequences begin degrading within 24–72 hours.
- pH drift: bacteriostatic water moves between 5.0 and 6.5 depending on the container and how long it stays open.
The rule: do not pre-fill a cartridge weeks ahead without empirical stability data for that peptide in that diluent. Pre-filling is a workflow convenience; treat it as one.
What the research community gets wrong about pre-filled pens
Most pen problems in the lab are hardware problems, not chemistry ones: a pen only behaves as well as the cartridge seated in it.
- Reading the dial as volume. The scale counts arbitrary units calibrated for one drug at one concentration — a relative counter, not a microliter readout, unless you verify displacement per click yourself.
- Assuming any 3 ml cartridge fits. Our pens take standard 3 ml (300-unit) glass cartridges with the 11 mm long plunger (stopper), the cartridge we sell. 3 ml cartridges are also made with a shorter, about 8 mm plunger, and a pen is built for one height, so check that a cartridge from another source has the 11 mm long plunger before loading it; the glass looks identical from the outside. Most suppliers never publish this fitment detail.
- Ignoring the needle interface. Our pens take universal 28G–33G screw-on pen needles. We have not tested other manufacturers' pens.
- Storing the pen horizontally. Extended liquid contact sits against the septum. Store tip-down so the air bubble rests at the plunger.
- Reusing one cartridge across peptides. Residual dead space carries into your next reconstitution and cannot be rinsed out through the septum. Use a new cartridge.
- Pre-filling with sterile water instead of bacteriostatic water. Sterile water carries no preservative, so a repeatedly pierced septum has nothing slowing microbial growth.
- Trusting a metal body to block all light. A metal barrel stops ambient light; a cartridge window does not.
A practical pre-fill protocol that wastes less
Goal: a cartridge setup that fills consistently and wastes less than 0.1 ml of sample. Work in this order:
- Prime the cartridge dry: push the plunger fully down, then pull it back to the fill mark. This seats it against the glass and breaks the static friction behind a sticky first dispense.
- Fill against back-pressure: use a blunt fill needle and syringe to push solution into the cartridge through the septum. Fill slowly, keeping a slight pull on the plunger rod to reduce bubbles at the plunger face.
- Expel the air immediately: invert the cartridge and push until a bead of liquid forms at the septum. That bead confirms the dead space is primed. Wipe, cap, store.
- Label on tape: peptide name, concentration, diluent and fill date. Not on the glass — condensation and solvents lift the ink.
Which cartridges actually fit
- Our pens take standard 3 ml (300-unit) glass cartridges with the 11 mm long plunger (stopper), the cartridge we sell. 3 ml cartridges are also made with a shorter, about 8 mm plunger, and a pen is built for one height, so check that a cartridge from another source has the 11 mm long plunger before loading it; the glass looks identical from the outside.
- They take universal 28G–33G screw-on pen needles.
- We have not tested other manufacturers' hardware, so we cannot say whether our cartridges work in third-party pens.
Frequently asked questions
How long does a peptide stay stable in a pre-filled cartridge?
In bacteriostatic water at 4°C, most peptides stay stable for 7 to 14 days; at room temperature, degradation begins within 24 to 72 hours. The exact window depends on the peptide sequence and the benzyl alcohol content of the bac water, so verify with your own stability data for protocols longer than a week.
Are metal peptide pens more accurate than plastic disposable pens?
Yes. Metal pens use a precision ratchet-driven plunger that maintains consistent displacement over hundreds of uses, while disposable plastic pens rely on spring-loaded gears that wear microscopically, causing 2-5% variation after 20 or more dispenses.
What is dead volume in a peptide cartridge and how do I minimize it?
Dead volume is the liquid trapped in the cartridge neck, septum, and crimp cap after the plunger bottoms out, typically 0.08 to 0.12 ml. Choose cartridges with flat or domed plunger faces and fill slowly against back-pressure to reduce the trapped air pocket that becomes dead volume.
Sources
- Bohannon, Postgrad Med 1999: Insulin delivery using pen devices. Simple-to-use tools may help young and old alike
- Jonker et al., Int J Clin Pharmacol Ther 2021: First pre-filled pen device with highly purified human menopausal gonadotropin (HP-hMG, Menopur) in liquid is shown to be bioequivalent to powder for reconstitution
- Lingen et al., Endocr Connect 2023: Advantages and disadvantages of connected insulin pens in diabetes management
- Bacteriostatic Water for Injection, USP: FDA label on DailyMed, U.S. National Library of Medicine (0.9% benzyl alcohol)
✔ Reviewed by Bryan Le, PharmD, RPh
Bryan is a licensed pharmacist (Doctor of Pharmacy, Registered Pharmacist). Reconstituting lyophilized preparations is core pharmacy practice, so he reviews The Lab’s content for technical accuracy and to keep it within a research-and-education scope, with no medical or dosing advice. View profile on LinkedIn.
Related from our lab: the pen · cartridges · bacteriostatic water · reconstitution calculators
Shared by PreppinPeppers for research, educational, and demonstration awareness only. We link to third-party coverage; we do not endorse it, and nothing here is medical advice or a recommendation to use any substance in humans or animals. Our products are sold for laboratory research use only.
Reminder: research and educational reference only. PreppinPeppers sells hardware and materials, not peptides. Not medical, dosing, or health advice, not evaluated by the FDA, and not intended for human or animal use.
