Mounjaro pen not working? What the leaflet says to check

If your Mounjaro pen is not working, the UK Instructions for Use cover a short list of faults: a stuck cap, a dose knob that stops before 1 or is stiff to push, no medicine at priming, or a dose window not back at 0. This guide gives that document's own answer to each and says where it is silent.

The sources are the UK KwikPen Instructions for Use (July 2025), the patient leaflet (August 2026) and the SmPC, all on the emc. Mounjaro (tirzepatide) is a prescription-only medicine; a prescriber decides whether it suits you, and this is information, not medical advice.

Mounjaro pen not working? Start with the step 2 checks

Before any troubleshooting, step 2 has you inspect the pen and lists the reasons not to use it at all: the name or strength on the label does not match your prescription; the pen is past its expiry date or looks damaged; the medicine has been frozen; or it has particles in it, is cloudy or is discoloured. Mounjaro should be colourless to slightly yellow. A pen that fails any of these is set aside and raised with your pharmacist or prescriber; the how-to-inject page covers the routine.

The dose knob will not turn to 1

First, the cap: the Instructions for Use FAQ says a cap that will not come off is gently twisted back and forth, then pulled straight off. Step 11 has you turn the knob until it stops and the 1 (the full 0.6 ml dose) shows. If it stops before the 1 appears, the Instructions for Use give one explanation and one instruction: there may not be enough medicine left for a full dose, so dispose of the pen, including what is left in it, and do not inject the remainder.

The Instructions for Use do not list a cap left on, a needle not yet attached or a dose already dialled as causes, and they never describe pulling the knob out. For a pen you know to be new, they offer no further cause, so stop and ask rather than force it.

The dose knob is hard to push

The same FAQ gives three possibilities:

• You are pushing too fast. Pushing the dose knob more slowly makes it easier to inject.
• The needle may be blocked. Put on a new pen needle and prime the pen again.
• Dust, food or liquid may be inside the pen. Throw that pen away and get a new one.

No medicine comes out when you prime

The pen counts as primed when a small amount of medicine comes out of the needle tip. If none does, the Instructions for Use give a fixed ladder: repeat steps 7 to 9 no more than two more times; if there is still nothing, change the pen needle and repeat steps 7 to 9 no more than one more time; if there is still nothing, contact your local Lilly affiliate (Lilly's UK company). There is no fourth try on the same needle. Step 8 assumes some air in the cartridge, and priming removes it; our air-bubbles guide covers that.

The dose window has not returned to 0

The 0 must be in the window before the needle comes out (step 12). Step 13 then says: if you do not see the 0, insert the needle back into your skin and finish the injection, and do not redial the dose. If you still do not think you received the full dose, do not start over or repeat the injection. Beyond that, the Instructions for Use say nothing about a dose you suspect was short; ask your pharmacist or prescriber, and the patient leaflet says to talk to your doctor immediately if you use more than you should. A drop on the needle tip afterwards is normal and does not affect your dose. Our double-dose guide covers the opposite mistake; the dosage page sets out the schedule.

Bent, blocked or damaged needles

The Instructions for Use treat the needle as disposable and the fix as a new needle: a blocked one is replaced and the pen primed again, a new one goes on for every injection (step 4, to help prevent infections and blocked needles), and the pen is never stored with one attached (step 16, to prevent leaking, a blocked needle and air entering the pen). A bent needle is not mentioned by name, and nothing in the document supports straightening or reusing one. Needles are not in the pack; ask your doctor, pharmacist or nurse which to use.

Frozen, left in heat or past its 30 days

Some pens are not working because they are no longer usable. A frozen pen is thrown away: the patient leaflet says “If the pen has been frozen, DO NOT USE”, and the SmPC says the same. Unused pens stay in the fridge at 2°C to 8°C. A pen in use 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, whichever comes first. None of the three emc documents covers a pen that may have gone above 30°C, such as one left in a hot car, so ask your pharmacist before using it. Our storage guide has the detail.

When to stop and ask

The Instructions for Use close with one line for everything else: if you have questions or problems with your Mounjaro KwikPen, contact your local Lilly affiliate or your doctor, pharmacist or nurse. Stop and ask if a pen fails the step 2 checks, will not dial to 1 when new, produces nothing after the permitted priming retries and a new needle, stays hard to push with a fresh needle, or leaves you unsure whether a full dose went in; never take a second injection to make up for one you think was short. Our FAQ page covers questions that are not faults.

Reporting a faulty pen: the MHRA Yellow Card scheme

The Yellow Card scheme is not only for side effects: the MHRA says anyone can report an issue with a medicine or a medical device through it, so a faulty pre-filled pen can be reported at yellowcard.mhra.gov.uk or through the Yellow Card app. Report side effects the same way; our side effects page lists the ones the patient leaflet describes.

Frequently asked questions

Related reading

• How the Mounjaro KwikPen works, click by click
• Air bubbles in a Mounjaro pen: when they matter
• Mounjaro storage fridge rules: 2–8°C and the 30-day exception