Practical guide

How to Fill and Use an Insulin Pen

A step-by-step reference for loading a reusable insulin pen with reconstituted peptide solution and delivering accurate subcutaneous doses: cartridge loading, priming, dose dialing, and sterile technique.

A reusable insulin pen is a multi-use injection device designed to accept standard 3 ml glass cartridges and dial doses in 1-unit increments. Researchers use them for peptide injection because they offer superior dose accuracy compared to drawing from a vial each time, reduce preparation time across frequent dosing schedules, and produce less material waste overall. GLP-1 peptide analogues, growth hormone secretagogues such as ipamorelin and GHRP-2, and other compounds dosed in small, precise volumes are well suited to reusable insulin pen for peptides delivery. This guide covers reusable pens loaded with a filled cartridge; it does not cover pre-filled disposable pens such as branded GLP-1 auto-injectors.

What You Need

Assemble all items before starting. Working with everything on a clean surface reduces handling time and minimises contamination risk.

Item	Notes
Reusable insulin pen device	Must be compatible with standard 3 ml cartridges. Examples include HumaPen Savvio, Owen Mumford Unifine, and comparable devices.
3 ml standard glass cartridge	Pre-filled with your reconstituted peptide solution. See the reconstitution guide for preparation technique.
Pen needles, 31–32g, 4–6 mm	ISO 11608-5 compliant pen needles fit all compliant pens. 4–6 mm at 31–32g is the standard recommendation for subcutaneous peptide pen injection.
Reconstituted peptide solution	Inspect for clarity before loading. Do not use solution that appears cloudy or contains visible particulates.
Alcohol swabs	70% isopropyl alcohol. Used to clean the injection site; allow to dry fully before inserting the needle.
Sharps container	Rigid, puncture-resistant container for immediate needle disposal after every injection.

Compatibility Note

Most reusable insulin pens are designed around the standard 3 ml glass cartridge format (ISO 11608-1). Before purchasing a pen for peptide use, confirm the manufacturer's specification explicitly lists 3 ml cartridge compatibility, some pens are designed only for proprietary cartridge formats used by specific insulin brands and will not accept universal cartridges.

Pen needles are standardised under ISO 11608-5. Any ISO-compliant pen needle will thread correctly onto any ISO-compliant pen regardless of manufacturer. For subcutaneous peptide injection, 31–32g and 4–6 mm is the standard recommendation: fine enough to minimise injection site trauma, short enough to reliably target the subcutaneous layer in most individuals without the risk of inadvertent intramuscular delivery.

Step-by-Step: Loading the Cartridge

Complete these steps before every new cartridge. Insulin pen cartridge loading takes under two minutes once the technique is familiar.

Inspect the pen and check the mechanism

Check that the pen body is undamaged, the dose window reads zero, and the cartridge holder is clear and free from residue. Confirm that the dose dial rotates freely and the injection button is not stuck. If the pen has not been used recently, cycle the mechanism once to ensure it is functioning correctly before loading a cartridge.

Remove the cartridge holder

Unscrew or detach the cartridge holder from the pen body according to the manufacturer's instructions for your specific pen model. Most pens twist off; some click. Set the cartridge holder on a clean surface with the open end facing up.

Load the filled cartridge

Insert the prepared 3 ml cartridge (filled with your reconstituted peptide solution) into the cartridge holder with the rubber plunger end going in first. The narrow metal-capped tip of the cartridge should face outward, toward the needle attachment end of the holder. Handle the cartridge by its body; do not touch the rubber stopper at the plunger end or the rubber septum at the tip.

Reattach the cartridge holder

Thread or click the cartridge holder back onto the pen body. You should feel or hear it seat fully. A partially attached holder will leak around the cartridge or misfire during injection, so take a moment to confirm full engagement before proceeding.

Attach a fresh pen needle

Remove the outer paper seal from a new pen needle, exposing the outer protective cap. Thread or click the needle onto the cartridge holder tip until firmly seated, do not overtighten to the point of cross-threading. Leave the inner needle cap in place until immediately before priming.

Prime to remove air

Remove the inner needle cap. Dial 2 units on the dose selector. Hold the pen with the needle pointing straight up and tap the cartridge body firmly several times with a finger to encourage any air bubbles to rise toward the needle tip. Press the injection button fully until it stops. A small droplet of solution should appear at the needle tip, this confirms the needle is clear and the pen mechanism is functioning correctly. If no droplet appears, re-dial 2 units and repeat the tap-and-press sequence until one does. Do not proceed to dose dialing until a droplet is confirmed.

How to fill an insulin pen cartridge: the cartridge itself is filled during the reconstitution process, a standard 3 ml glass cartridge is filled with the desired volume of reconstituted peptide solution using a syringe and needle before being loaded into the pen. See the reconstitution guide for the preparation steps.

Step-by-Step: Setting and Delivering the Dose

Dial the correct dose

Rotate the dose selector until the dose window shows the required number of pen units. For U-100 cartridges, the standard, each unit corresponds to 0.01 ml (10 µl) of solution. The dose accuracy section below explains how to convert a target dose in mcg to pen units. You can also consult the syringes guide for the underlying U-100 arithmetic, which is identical whether you are using a syringe or a pen.

Swab the injection site and let dry

Wipe the chosen injection site with an alcohol swab using a circular outward motion. Allow the skin to dry completely for 10–15 seconds before inserting the needle. Inserting through wet alcohol introduces isopropyl into the tissue, causes stinging, and may carry surface bacteria inward with the needle. For injection site selection and rotation guidance, see the injection technique guide.

Insert the needle

For most individuals, insert the needle at 90° to the skin surface. For very lean individuals with minimal subcutaneous fat, a 45° angle reduces the risk of inadvertent intramuscular placement. Insert with a single smooth, decisive motion, hesitant or slow insertion increases discomfort. For 4–6 mm pen needles a skin pinch is generally not required, though a light pinch is acceptable for very lean individuals.

Press the button fully and hold for 10 seconds

Depress the injection button until it stops. Then hold the pen still, with the needle fully inserted, for a complete 10 seconds before withdrawing. This step is critical and is the most common source of incomplete dosing with pen devices: unlike a syringe plunger where delivery is immediate, the pen mechanism continues to express solution for a moment after the button reaches its stop. Withdrawing the needle too quickly leaves part of the dose in the needle hub rather than in the tissue.

Remove and dispose of the needle immediately

Withdraw the needle at the same angle it was inserted. Unscrew or detach the needle from the pen and discard it directly into a sharps container in a single motion, do not recap. Apply a fresh alcohol swab or dry gauze lightly to the injection site if needed; do not rub.

After Use

Always remove the pen needle after every injection. Leaving a needle attached creates a direct air pathway into the cartridge. As the pen cools after injection, air is drawn in through the needle, displacing solution. This leads to air accumulation in the cartridge, dose inaccuracy on subsequent uses, and a contamination pathway into the cartridge contents.
Store at the correct temperature for your reconstituted compound. Most reconstituted peptides require refrigeration at 2–8°C. Check the specific compound's storage requirements. The pen itself can remain at room temperature between sessions if the cartridge duration is short, but return it to the fridge if storing for longer periods.
Replace the pen cap between uses. The cap protects the cartridge tip from light exposure and reduces contamination risk at the needle attachment point.

Dose Accuracy: Converting mcg to Pen Units

Reusable insulin pens dial in integer units. For U-100 cartridges, the standard format, 1 unit equals 0.01 ml (10 µl). The conversion from a target peptide dose in mcg to pen units follows the same U-100 logic used for insulin syringes:

Formula: pen units = (target dose in mcg ÷ concentration in mcg/ml) × 100

Example: cartridge filled at 1000 mcg/ml, target dose 250 mcg
pen units = (250 ÷ 1000) × 100 = 25 units

Because pen dials only move in whole units, doses that fall between unit marks require rounding. Always round to the nearest whole unit, and factor this into concentration selection at the reconstitution stage, a higher concentration allows finer dose resolution per unit. Use the dose calculator to compute the exact pen units for any dose and concentration combination.

Troubleshooting Quick Reference

Problem	Likely cause	Resolution
Air bubbles visible in cartridge	Normal after cartridge loading or storage without needle removed	Prime again: dial 2 units, needle pointing up, tap cartridge firmly, press injection button fully
Dose dial will not turn	Pen at maximum dose limit, or cartridge nearly empty with insufficient volume remaining for dialled dose	Check remaining cartridge volume; if at max, re-zero and re-dial; if cartridge is nearly empty, replace with a fresh filled cartridge
No droplet appears during priming	Air column too large, needle not properly seated, or blocked needle tip	Prime 2–3 more times with needle firmly pointing upward; check that the needle is fully threaded; if still no droplet, replace the pen needle
Leaking around the needle hub	Needle not fully threaded onto cartridge holder tip	Remove the needle and reattach firmly; do not overtighten to the point of stripping the thread; use a fresh needle if the seal appears damaged

Frequently Asked Questions

Can I use any insulin pen for peptides?

Not all pens are interchangeable. You need a reusable insulin pen explicitly compatible with standard 3 ml glass cartridges. Some pens are designed only for proprietary cartridge formats tied to a specific insulin brand and will not accept universal cartridges. Confirm compatibility in the manufacturer's specification before purchasing. Pen needles are standardised and compatible across all ISO 11608-5 compliant pens.

How do I calculate my dose in pen units?

For U-100 cartridges, 1 unit = 0.01 ml. Use: pen units = (target dose mcg ÷ concentration mcg/ml) × 100. A 250 mcg dose from a 1000 mcg/ml cartridge = 25 units. Use the dose calculator for any combination.

How long can I store a filled cartridge?

Storage duration depends on the specific reconstituted compound. Most reconstituted peptides remain stable for 4–6 weeks when refrigerated at 2–8°C with bacteriostatic water. Always inspect the solution for cloudiness or particulates before each use.

Why must I remove the pen needle after every injection?

Leaving a needle attached creates a direct air pathway into the cartridge. As the pen cools post-injection, air is drawn in through the needle tip, displacing solution and creating an air column. Subsequent injections will then deliver a partial dose, part air, part solution, and the contamination pathway increases with each cycle.

Do I need to prime the pen before every injection?

Yes, whenever a fresh needle is attached, which should be before every injection. Attaching a new needle introduces a small air gap in the hub. Priming (dial 2 units, needle up, tap, press until droplet appears) clears this air and confirms the needle is patent before you dial your dose. Skipping the prime after attaching a fresh needle results in the dialled dose being partially delivered as air rather than solution.

Key Takeaways

A reusable insulin pen offers better dose accuracy and less prep time than drawing from a vial for each injection, making it practical for frequent dosing schedules with GLP-1 analogues, GHSs, and other small-volume peptides.
Confirm 3 ml cartridge compatibility before purchasing a pen; not all pens accept universal cartridges. Pen needles (ISO 11608-5) are interchangeable across compliant pens.
Always prime with 2 units (needle pointing up, tap, press) after attaching a fresh needle, required before every injection to clear the air gap in the needle hub.
Hold the injection button for a full 10 seconds after full depression before withdrawing the needle. This is the single most commonly missed step and the primary cause of incomplete dose delivery with pen devices.
Remove and dispose of the pen needle immediately after every injection. Never store the pen with a needle attached, doing so causes air ingress, dose inaccuracy, and contamination risk.
For U-100 cartridges, 1 pen unit = 0.01 ml. Pen units = (dose mcg ÷ concentration mcg/ml) × 100.
Rotate injection sites systematically to prevent lipohypertrophy from repeated injection at the same location.

Dose Calculator