How to use a peptide pen

A “peptide pen” is usually a reusable insulin-style pen with a cartridge of reconstituted peptide in it. The pen does not know what is in the cartridge. It measures volume. Everything about getting the right dose follows from that. This guide walks through the steps the pen makers' own manuals share and the places where peptide use departs from them.

By OptiPin Editorial·Last updated October 2026

Standard cartridge

3 mL = 300 units

Insulin-pen prime

2 units

Hold after pressing

5 to 6 seconds

Needles

New every time

Get care the same day for redness, heat or swelling that spreads from an injection site, or a fever after injecting. Call emergency services for swelling of the face or throat or trouble breathing after an injection. On insulin, or a GLP-1 taken alongside diabetes medication, shakiness, sweating or confusion can mean low blood sugar. Never share a pen with anyone, even with a new needle.

TL;DR

• The dial measures volume. On a pen built for 100 units/mL cartridges, one unit is 0.01 mL whatever is dissolved in it. Your dose in units depends on the concentration. The peptide pen calculator does the conversion.
• Prime, dial, inject, hold, remove. Lilly and Novo Nordisk both describe a 2-unit prime before each injection and a 5 to 6 second hold after the button bottoms out.
• Read the window, not the clicks. Lilly's manual warns that counting clicks is how a wrong dose gets dialed.
• New needle every time. Take it off afterward. A needle left on can leak, block or let air into the cartridge.
• Filling your own cartridge is outside every pen manual. That step is where contamination and concentration errors come in.

Three different things called a pen

Type	What the dial means	Who fills it
Branded prefilled pen Ozempic, Mounjaro KwikPen, Norditropin FlexPro	Calibrated for that product's own doses. The dial is not a general unit scale.	The manufacturer. See the GLP-1 click chart for those pens.
Reusable pen with factory cartridges HumaPen Savvio, NovoPen Echo	Units of 0.01 mL. Savvio steps 1 unit up to 60. Echo steps half a unit up to 30.	The manufacturer fills the cartridge. You load it into the pen.
Reusable pen with a cartridge you filled What most people mean by “peptide pen”	Units of 0.01 mL, if the pen is U-100 scaled. The dose per unit is whatever your mixing produced.	You, or a compounding pharmacy. No pen manual covers this step.

Lilly says its Humalog cartridges should be used only in the pen it names, because it has not verified dosing accuracy with other pens. That is worth holding onto when a pen and a cartridge come from different places. Nobody has checked that combination for you.

Step by step

These are the steps Lilly's KwikPen manual and Novo Nordisk's NovoPen Echo guide share. Your own pen's manual overrides anything here.

Check the cartridge. Right medicine, right strength, no cracks. The liquid should look as it should for that medicine. A cloudy or discolored solution that is meant to be clear is a reason to stop.
Load it. Unscrew the cartridge holder. If the piston rod sticks out, push it back in until it stops. Slide the cartridge in and screw the holder back on until you hear or feel a click.
Attach a new needle. Screw it on, pull off the outer cap and keep it, then throw away the inner cap.
Prime. Dial 2 units, hold the pen needle up, tap the cartridge so air rises and press the button until the window reads 0. A drop should appear at the tip. If it does not, prime again. Lilly says to change the needle if four primes produce nothing.
Dial the dose. Turn the dial until the dose window shows your number. You can turn it either way to correct it. Lilly's KwikPen will not let you dial more than is left in the cartridge, so check whether yours does the same.
Inject and hold. Insert the needle and press the button until the window shows 0. Then keep the needle in: Lilly says count slowly to 5, Novo Nordisk to 6. The hold is part of getting the full dose. Lilly adds that if the window does not read 0, you have not had the full amount.
Remove the needle. Put the outer cap back on, unscrew the needle and drop it in a sharps container.
Cap and store the pen. Without a needle on it. See storage below.

For where to inject and how to rotate sites, see the injection guide.

Dialing: units, half-units and mg

On a pen built for 100 units/mL cartridges, the dial counts hundredths of a mL. Lilly's KwikPen manual spells it out: 3 mL, 300 units, 100 units/mL. Fill that cartridge with 10 mg of peptide in 2 mL and each unit carries 50 mcg. Fill it with the same 10 mg in 3 mL and each unit carries about 33 mcg. The pen dials the same 5 units either way. The first dose is 50% larger than the second. The calculator works out units from your own concentration.

The step size matters too. A dose that works out at 7.5 units cannot be dialed on a 1-unit pen. It can on a half-unit pen like the NovoPen Echo. Branded growth hormone pens skip units altogether: Norditropin FlexPro 10 mg/1.5 mL dials in 0.05 mg steps up to 4.0 mg.

Filling a cartridge yourself: what the manuals do not cover

Every pen manual assumes a cartridge filled and sealed by a manufacturer. Moving a reconstituted peptide from a vial into an empty cartridge is not described in any of them. It is the riskiest part of using a peptide pen:

Contamination. Each transfer is another chance to introduce bacteria into a container you will inject from for weeks. Compounding pharmacies work to sterile-compounding standards (USP <797> in the US) that a kitchen table does not meet.
Concentration. The dose per unit is set by how much liquid went into the vial, not the cartridge. Write the concentration on the cartridge. A pen full of unlabeled liquid is how a 2 mL dilution gets dialed as if it were 3 mL.
Air. A cartridge with a large air gap delivers less than the dial says until the gap is primed out. That is what the prime is for. It matters more on a hand-filled cartridge.

If a pharmacy can dispense the cartridge ready-filled and labeled, that removes most of these problems. If you are filling cartridges yourself, it is a conversation to have with whoever prescribed the peptide.

Storage and how long a cartridge lasts

Store the pen the way the medicine in it needs, because the pen has no shelf life of its own. The manuals show how much that varies:

Insulin (Lilly KwikPen). Unused pens in the fridge at 2–8 °C. The pen in use at room temperature below 30 °C, thrown away after the period in the leaflet even if insulin is left.
Growth hormone (Norditropin FlexPro 10 mg/1.5 mL). In use, up to 4 weeks in the fridge or 3 weeks below 25 °C. Not after freezing. Not if the solution is cloudy or discolored.
A reconstituted peptide. Commonly treated as good for about 28 days refrigerated. Some compounds degrade faster. The storage guide explains why the figure varies.

That last point decides how big a cartridge to fill. If the dial says the cartridge holds 40 doses and you inject daily, the calendar ends it before the dial does. The calculator flags this. Keep the needle off between doses: Lilly's manual says a needle left on can leak, block or let air into the pen.

Pen or insulin syringe?

	Pen	Insulin syringe
Measuring the dose	Dial and read the number in the window.	Draw to a line on the barrel. Easy to misread small volumes.
Waste per dose	The prime, often 2 units every injection.	Very little with a fixed needle.
Filling	Factory cartridges are sealed. Self-filled cartridges add a transfer step.	Drawn straight from the vial each time.
Smallest step	1 or 0.5 units, depending on the pen.	Depends on the syringe's markings.
Best fit	Doses large enough that the prime is a small share.	Very small doses, where a 2-unit prime would be a large share.

The prime is the deciding factor more often than people expect. On a 5-unit dose, a 2-unit prime means a 3 mL cartridge gives 42 doses rather than 60. On a 40-unit dose it barely matters.

Common mistakes

Using another pen's click chart. A chart is only valid for the pen and strength it was written for.
Counting clicks instead of reading the window. Lilly's manual says this outright.
Skipping the prime with a new needle. Lilly: without it you may get too much or too little.
Pulling the needle out as soon as the button bottoms out. The manuals ask for a count of 5 or 6.
Leaving the needle on between doses. Leaks, blocked needles and air in the cartridge.
Sharing a pen. Lilly's manual says not to, even with a new needle, because infection can pass between users.
Assuming a self-filled cartridge lasts as long as a factory one. The solution's own stability sets the limit.

Know when the cartridge runs out

Log every pen dose in OptiPin

OptiPin counts your vials and pens, projects the run-out date from the doses you actually log and warns you before you're low. All on-device, no account.

Download on the App Store

Frequently asked questions

What is a peptide pen?

Usually a reusable pen injector of the kind built for 3 mL insulin cartridges, used with a cartridge holding a reconstituted peptide. The dial measures volume on the insulin scale, where one unit is 0.01 mL, so the dose it delivers depends entirely on the concentration in the cartridge. Branded prefilled pens such as Ozempic or Norditropin FlexPro are a different thing: the manufacturer fills them and the dial is calibrated for that product.

Do I need to prime a pen before every injection?

Insulin pen manuals from Lilly and Novo Nordisk say yes: a 2-unit prime before each injection, to clear air from the new needle. Some branded growth hormone pens differ. Norditropin FlexPro asks for a flow check before the first injection from each new pen. Follow the manual for the pen you have. Count the prime when you work out how many doses a cartridge holds.

Can I reuse pen needles?

Lilly's KwikPen manual says to use a new needle for each injection, to help prevent infections and blocked needles. It also says not to store the pen with a needle attached, because the needle can leak, block or let air into the cartridge.

Can two people share one pen if they change the needle?

No. Lilly's manual says not to share a pen even if the needle has been changed, because infection can pass between users. A new needle does not make sharing safe.

How long can a peptide cartridge stay in the pen?

As long as the medicine in it stays usable, which is set by the medicine and not the pen. A reconstituted peptide is commonly treated as good for about 28 days refrigerated. Some compounds degrade faster. A branded pen states its own in-use period: Norditropin FlexPro 10 mg/1.5 ml allows up to 4 weeks refrigerated or 3 weeks below 25 °C.

Educational, not medical advice. This page describes how pen injectors work, drawn from the manufacturers' own instructions. It does not give doses or recommend any medicine. It does not replace the manual for your pen or the instructions of the person who prescribed what is in it. Many peptides used in pens are not approved medicines. OptiPin is a tracking tool, not a prescriber.

Peptide pen calculator · Peptide reconstitution calculator · GLP-1 pen click chart · HGH reconstitution · Peptide storage · Injection guide

Sources

Eli Lilly. Humalog 100 units/ml KwikPen user manual, electronic medicines compendium. Cartridge capacity, the 2-unit prime and count to 5, reading the window rather than clicks, a new needle each time, needle off for storage, no sharing, in-use storage.
Novo Nordisk. How to use NovoPen Echo, quick guide. Cartridge loading, half-unit steps to 30 units, the 2-unit flow test, count to 6, needle removal after each injection.
Novo Nordisk. Norditropin FlexPro 10 mg/1.5 ml package leaflet, electronic medicines compendium. mg dosing in 0.05 mg steps, flow check before a new pen's first injection, in-use storage.
United States Pharmacopeia. General Chapter <797> Pharmaceutical Compounding: Sterile Preparations. The standard US compounding pharmacies work to.
Lilly Medical. What pen should be used for Humalog cartridges. HumaPen Savvio's 1 to 60 unit range and Lilly's position on unverified pen and cartridge combinations.