Air bubbles in a Mounjaro pen: when they matter

Air bubbles in a Mounjaro pen are a common worry: you hold the cartridge up to the light, see a small pocket of air, and wonder whether this week's dose is still right. The UK Instructions for Use treat a bubble as expected rather than a fault, and the priming step exists to deal with it.

This guide sticks to the UK documents: the KwikPen Instructions for Use (last revised July 2025) and the patient leaflet (last revised August 2026), both on the emc. Mounjaro (tirzepatide) is a prescription-only medicine, a prescriber decides whether it suits you, and this is information, not medical advice.

Are air bubbles in a Mounjaro pen normal?

The Instructions for Use never describe a bubble in the cartridge as a defect. Two steps assume one may be there. Step 8 reads: “Hold your pen with the needle pointing up. Tap the cartridge holder gently to collect air bubbles at the top.” Step 9 explains why: “Priming removes air from the cartridge and makes sure that your pen is working correctly.”

So air is something the routine handles. The document gives no acceptable bubble size, and it does not say a bubble is harmless if you skip priming. The leaflet's reasons not to use a pen are different: medicine that is cloudy, discoloured or has particles in it, a damaged pen, or one that has been frozen. Mounjaro should be clear and colourless to slightly yellow. A bubble is not on that list.

Priming: the “preparing to inject” steps

Priming sits inside one sequence headed “Preparing to inject Mounjaro KwikPen”, which runs from washing your hands (step 1) through attaching a new needle (steps 4 to 6) to the prime itself:

1. Step 7: slowly turn the dose knob until you hear 2 clicks and the extended line shows in the dose window: “the prime position”.
2. Step 8: needle pointing up, tap the cartridge holder gently so any air collects at the top.
3. Step 9: push the dose knob in until it stops and slowly count to 5 while holding it, releasing some medicine into the air. “Your pen has been primed if a small amount of medicine comes out of the tip of the pen needle.”

Is priming only for a new pen?

Many people assume you prime once, when the pen is new, and skip it for the remaining three doses. The UK Instructions for Use make no such distinction: one set of steps for each injection, with a new pen needle every time (step 4 says this helps “prevent infections and blocked needles”), then the prime, then the dose. Our first-injection walkthrough puts the prime in context, and the how-to-inject page covers the pen more broadly.

What to do if no medicine comes out

Sometimes you tap, prime and nothing appears at the needle tip. The Instructions for Use give a fixed ladder:

1. Repeat steps 7 to 9 “no more than 2 additional times”.
2. If you still see no medicine, change the pen needle and repeat steps 7 to 9 “no more than 1 additional time”.
3. If there is still nothing, the document says to contact your local Lilly affiliate (the manufacturer). Its closing note adds your doctor, pharmacist or nurse for any problem with the pen.

The fresh needle is there because of the document's own troubleshooting list: if the dose knob is hard to push, “your needle may be blocked”, and the fix is to “put on a new pen needle and prime the pen”. The ladder never asks you to dial a larger amount or keep pressing beyond the counted hold.

Why you should not try to clear bubbles another way

Flicking the pen hard, shaking it, or expelling a full dose into the air appear nowhere in the Mounjaro KwikPen Instructions for Use, and there are concrete reasons to stick to the printed method.

• The pen holds four fixed doses of 0.6 ml, so repeated priming can use up medicine the pen needs for those doses. The document's FAQ spells out the consequence: if you cannot turn the dose knob until the full-dose symbol shows, “there may not be enough medicine left in the pen to give a full dose”: dispose of the pen rather than inject what is left.
• Doubling up is ruled out. If you are unsure the full dose went in, check the dose window; if the symbol is not there, put the needle back in and finish. “Do not redial the dose”, and if you still doubt it, “do not start over or repeat the injection”. The leaflet adds that if you use more Mounjaro than you should, talk to your doctor immediately.

Our dosage page explains how the strengths step up, and the missed-dose guide covers timing if a dose is lost.

Storage and air: what the leaflet does and does not say

The UK leaflet and Instructions for Use make no link between storage temperature and air in the cartridge. Their rules are short: unused pens stay in the fridge at 2°C to 8°C and must not be frozen (a frozen pen is thrown away); a pen you have started may be kept at up to 30°C, away from heat, and is disposed of 30 days after first use or after its fourth weekly dose.

The one place the Instructions for Use mention air getting into the pen is step 16, on removing the needle after each injection: “Do not store the pen with the needle attached to prevent leaking, blocking the needle, and air from entering the pen.” If you have been leaving the needle on between doses, that is the habit to change first. Our storage guide covers fridge rules, travel and frozen pens in full.

When to contact your pharmacist or prescriber

The leaflet's general rule is: “Check with your doctor or pharmacist if you are not sure how to use this medicine.” Get in touch if no medicine appears after the permitted retries and a new needle; if the dose knob will not turn to the full-dose symbol; if the pen looks damaged, cloudy, discoloured or has particles, or may have been frozen; or if you are unsure whether a full dose went in. People with vision problems should not use the pen without help from someone trained to use it.

If you notice side effects, tell your pharmacist or prescriber and report them through the MHRA Yellow Card scheme. The leaflet lists injection-site reactions such as itching or redness as common (up to 1 in 10) and injection-site pain as uncommon (up to 1 in 100). Our side effects page has the full list, and this guide covers technique when the injection stings.

Frequently asked questions

Related reading

• How to inject Mounjaro the first time: a calm step-by-step
• Mounjaro storage fridge rules: 2–8°C and the 30-day exception
• Does the Mounjaro injection hurt? Technique that helps