What it is
A refillable GLP-1 research pen is a reusable precision click-dial device that holds a standard 3 mL glass cartridge and measures liquid on the U-100 scale, meaning 100 units equal 1 mL.
In this article
Community roundup: we summarize and fact-check what the peptide community discusses on Reddit and research forums, then add the exact reconstitution math and handling notes.
Reddit threads about refillable pens and 3 mL cartridges get a lot of attention and are mostly accurate. Here is a fact-checked summary for anyone comparing notes at the bench.
Why 3 mL cartridges
Standard precision click-dial pens are designed around the 3 mL glass cartridge, which is why most reconstitution math targets that volume. Many researchers pair a reusable metal pen with a glass cartridge for repeatable, precise draws.
Cartridge fit is not universal, however. 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. This tolerance detail is rarely published elsewhere, and it is the first thing to check when sourcing third-party cartridges.
Needle choice stays flexible. The pen takes any universal 28G–33G screw-on pen needle, so gauge selection is independent of the cartridge body.
Units and dialing
A precision click-dial pen measures each turn of the dial in U-100 units. U-100 means 100 units occupy every 1 mL of liquid inside the cartridge. Think of units as tick marks on a ruler: the mechanism clicks from one mark to the next, so each setting lands on the same spot.
For the dial to engage, the cartridge must seat flush against the pen body. 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 universal 28G–33G screw-on types. See the U-100 units explainer for the full math.
Planning refills
Refilling starts with matching the cartridge to the pen, not the other way around. Get the cartridge choice right once and the rest of the schedule is straightforward — no guesswork on cartridge life or timing.
Which cartridges fit our pens
- 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.
- Pair the pen with universal 28G–33G screw-on pen needles.
Enter your cartridge details in the peptide calculator and it builds a refill schedule automatically for a single compound or a blend. You see how often each cartridge needs replacing and when to prepare the next one, so the next cartridge is ready before the current one runs dry.
Standard precision click-dial pens are designed around the 3 mL glass cartridge, which is why most reconstitution math targets that volume.
Related reading
- How A Dial A Dose Pen Works
- 3Ml Glass Cartridges Explained
- Peptide Reference Library
- peptide calculator
Reminder: research and educational reference only. Preppin Peppers 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.
Frequently asked questions
Why do reconstitution pens use 3 mL cartridges?
Precision click-dial pens are engineered to accept standard 3 mL glass cartridges, so reconstitution math and refill schedules are conventionally built around that volume for precision and reproducibility.
What does U-100 mean on a peptide research pen?
U-100 means 100 units fit in every 1 mL of liquid. Units act like ruler tick marks, letting researchers dial to the same position each draw for consistent, repeatable volume delivery.
How do I calculate how many draws a 3 mL cartridge holds?
Divide total cartridge volume by units per draw (3 mL = 300 U-100 units). A peptide calculator planner can automate this and generate a full refill schedule from a single compound or blend entry.
More in our peptide pens collection.
What the research community gets wrong about refilling a research pen with 3 mL cartridges
Most bench notes on refillable pens are close, but a few ideas get repeated on forums that do not hold up on the bench. Here is what to keep straight when you load 3 mL cartridges.
- A "unit" is a measure of liquid, not a measure of compound. On the U-100 scale, one unit is one hundredth of a milliliter. The pen only moves the plunger a set distance per click and has no idea how much peptide is dissolved, so the same unit mark holds a different amount of compound depending on how you reconstituted the cartridge.
- U-100 is a scale printed on the pen, not the strength of what is inside. The pen counts volume the same way no matter what you loaded. Your reconstitution math sets the amount per unit, not the pen.
- A 3 mL cartridge does not give you 3 mL of usable draws. Dead space near the plunger and stopper means the last bit may not draw cleanly. Plan refills around the volume you can actually pull, not the label.
- "Fits" is not the same as "seats correctly." Capacity is not the same as compatibility. The hidden variable is the stopper length: 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. Almost no supplier calls this out.
- A compatible needle is not a compatible cartridge. Many posts treat the two as one problem. The pen takes universal 28G–33G screw-on pen needles, so the needle side is forgiving. The cartridge side depends on diameter, length, and stopper geometry, and is not.
- Reusable does not mean maintenance free. Residue, trapped air, a worn lead screw, or a stretched spring all change how repeatable draws are over time. Consistent results come from clean handling and inspection, not from the hardware alone.
From our bench: Measure the volume you can actually draw from a filled cartridge before the plunger bottoms out, and count the full draws at your chosen unit setting. Send the cartridge brand, fill volume, and draw count. Real numbers help other researchers plan refills, and we will not publish invented figures.
Sources
- NovoLog (insulin aspart) , FDA/DailyMed label confirming 100 units/mL (U-100) and 3 mL PenFill cartridge delivery device
- Humalog (insulin lispro) , FDA/DailyMed label confirming 100 units/mL (U-100) and 3 mL single-patient-use cartridges
- 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
✔ 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.
