Fake Mounjaro pens: the MHRA alert and how to spot a counterfeit

In short

MHRA Drug Safety Update published 24 February 2026: falsified Mounjaro KwikPen 15mg pre-filled pens
Batch D873576 in the 15mg strength is falsified. The same batch number on a 7.5mg pen is genuine
The expiry date (10/26) does not help identify the fakes. Check the dose and batch number only
Five pens had been identified at publication. In all but one, the dose knob came off during use
Testing found tirzepatide in the pens, but the MHRA says sterility cannot be assured
Report a suspected fake through the Yellow Card scheme or to info@mhra.gov.uk

The MHRA confirmed falsified Mounjaro KwikPen 15mg pens in the UK on 24 February 2026. The fakes carry batch number D873576. Every 15mg pen with this batch number is falsified, while genuine 7.5mg pens use the same number and are safe. If you have a 15mg pen from batch D873576, do not inject from it. Keep it, and email info@mhra.gov.uk with "Mounjaro Pens" in the subject line.

What the MHRA found

The warning appeared in the MHRA Drug Safety Update for February 2026, under the title "Falsified Mounjaro KwikPen 15mg pre-filled pens". At the time of publication the MHRA had identified five affected pens.

The problem came to light through the pens themselves. In all but one case the dose knob came off while the person was using the pen. All the identified pens had a faulty mechanism. Laboratory testing showed the pens did contain tirzepatide, the active ingredient in Mounjaro. The MHRA was unable to confirm the contents were sterile, which means a risk of infection or an allergic reaction.

The MHRA said it had no reports of patient harm needing treatment and rated the risk as low on the cases reviewed so far.

Detail	What the alert says
Product	Mounjaro (tirzepatide) KwikPen 15mg pre-filled pen
Batch number	D873576
Expiry on the fake pack	10/26, not useful for spotting the fake
Other strengths	Genuine 7.5mg pens use batch D873576 and are not affected
Pens found at publication	5
Fault reported	Dose knob came off during use, faulty mechanism
Contents	Tirzepatide present, sterility not assured

Why the batch number looks real

Eli Lilly, the maker of Mounjaro, checked the packs. The batch number and expiry date printed on the fakes match a genuine Mounjaro KwikPen 7.5mg batch. Whoever made the fakes copied real codes from a lower strength and printed them on 15mg packaging.

This is why the expiry date tells you nothing here. A 7.5mg pen from batch D873576 with an expiry of 10/26 is a genuine product. A 15mg pen with the same codes is not. The dose and the batch number together are the test.

Where the pens came from

The MHRA traced the fake pens to a single online pharmacy in the UK. The General Pharmaceutical Council (GPhC), which regulates pharmacies in Great Britain, served a notice of conditions on the business. The MHRA said its investigation was ongoing.

Fakes reaching patients through a registered pharmacy is unusual. Most falsified weight loss medicines in the UK come through unregulated sellers. The MHRA guidance on GLP-1 medicines says they should not be bought from beauty salons or through social media. The same guidance warns of counterfeit pre-filled pens made to closely mimic licensed products, and says powder in vials for mixing before injection is not an authorised form of any GLP-1 medicine.

How to check your pen

Start with the alert, then run the general checks.

Find the strength and batch number on the outer carton. Check the same details on the pen label.
If the strength is 15mg and the batch is D873576, stop. Do not use the pen.
Check the strength and batch on the carton match the pen inside. A mismatch is a reason to stop and ask your pharmacy.
Look at the liquid through the window. The patient leaflet describes Mounjaro as clear and colourless to slightly yellow. Do not use it if it looks cloudy, discoloured or has particles in it.
Check the pen body. The leaflet says not to use a pen which looks damaged.
Check the dose knob turns and clicks as the instructions describe. A loose or detached knob was the warning sign in every case but one.
Check the pack came from a pharmacy on the GPhC register, with a prescription in your name.

The guide on how to use the Mounjaro KwikPen shows what normal use looks and sounds like, which makes a fault easier to spot. Lilly is also bringing in a modified version of the pen with a different plunger position. A genuine pen of either design is fine, and the page on KwikPen changes in 2026 explains how to tell them apart.

What to do if you have one

The MHRA advice for patients is plain.

Do not use the pen, and do not try to draw any dose out of a faulty pen with a syringe.
Keep the pen somewhere safe. The MHRA will arrange to collect it.
Email info@mhra.gov.uk with "Mounjaro Pens" in the subject line.
Report it through the Yellow Card scheme, online or through the Yellow Card app.
Contact your prescriber or pharmacy about a replacement. The MHRA told healthcare professionals some patients will need new medication to keep treatment going.

If you have missed a dose while you sort out a replacement, follow the normal missed dose rule. Take the dose within 4 days (96 hours) of the usual time. After 4 days, skip it and take the next dose on your usual day. If you have been off treatment for longer, ask your prescriber which dose to restart on. The guide on restarting Mounjaro after a break covers this.

If you have already injected from a fake pen

The main risk the MHRA names is a lack of sterility. An unsterile injection can cause a skin infection or, rarely, a wider infection. The MHRA asked healthcare professionals to check anyone who may have used a fake pen for signs of infection, sepsis or allergic reaction.

Call 999 or go to A&E if you have:

swelling of your lips, mouth, throat or tongue, or trouble breathing
a fast heartbeat with a high or very low temperature, confusion, or skin which looks mottled or blue
feeling seriously unwell after an injection and getting worse fast

Call 111 or see a GP the same day if the injection site turns red, hot, swollen or painful and the area is spreading, or if you develop a high temperature. The page on allergic reactions to Mounjaro sets out the signs in more detail.

Mention the batch number to whoever treats you. The fakes contained tirzepatide, so the usual side effects of Mounjaro, such as nausea, are also possible.

How to lower your risk next time

The MHRA says the only way to guarantee a genuine GLP-1 medicine is to get it from a legitimate pharmacy on a prescription. In Great Britain you can search the GPhC register online by pharmacy name or premises number. In Northern Ireland the Pharmaceutical Society of Northern Ireland keeps the register.

Signs a seller is not legitimate:

no consultation or health questions before supply
sales through social media messages or beauty salons
powder in vials, or pens sent without a pharmacy label
no named pharmacist or GPhC registration number on the website

The guide on how to buy Mounjaro in the UK safely walks through the full process. If you change supplier, the page on switching pharmacy covers what to pass on so your dose and records stay right. To compare prices from registered sellers, see UK Mounjaro prices.

Reporting through Yellow Card

The Yellow Card scheme is the MHRA route for reporting suspected side effects, suspected fake medicines and faulty pens. You can report online at yellowcard.mhra.gov.uk or through the Yellow Card app on the Apple App Store or Google Play. Healthcare professionals can also report from GP and hospital IT systems.

A report does not need proof. A pen which feels wrong, a dose knob which comes loose or a pack with mismatched details is enough reason to report. Mounjaro carries a black triangle, which means the MHRA monitors it more closely and wants every suspected problem reported. The safety page lists the other checks to run on every new pen.

Questions people ask

How do I know if my Mounjaro pen is fake?

For the February 2026 alert, check two things on the carton and pen label: the strength and the batch number. A 15mg pen with batch D873576 is falsified. Any pen with a faulty dose knob, a damaged body or cloudy liquid should not be used, whatever its batch, and should be reported to the MHRA.

Is batch D873576 always fake?

Only in the 15mg strength. Lilly confirmed D873576 is a genuine batch number for Mounjaro KwikPen 7.5mg, and those pens are not affected. The fakes copied a real batch number and expiry from a lower strength.

I injected from a fake pen. What should I do?

Stop using the pen and keep it. The MHRA advises seeking immediate medical attention for any unusual symptoms after an injection, because sterility cannot be assured. Watch for redness, heat or swelling at the injection site, a high temperature, or signs of an allergic reaction such as swelling of the lips or face.

Where did the fake Mounjaro pens come from?

The MHRA traced the pens to one UK online pharmacy. The General Pharmaceutical Council placed conditions on the pharmacy and the MHRA investigation continued. Separately, the MHRA warns against buying GLP-1 medicines from social media sellers or beauty salons.

Can I get a replacement pen?

The MHRA told healthcare professionals some patients may need replacement medication to continue treatment. Contact your prescriber or the pharmacy you use for a new supply. Ask them to check the dose and batch before they send it.

Should I worry about other Mounjaro batches?

The February 2026 alert named one batch in one strength. The MHRA also warns of counterfeit pre-filled pens designed to look like licensed products, so buy only on a prescription from a GPhC-registered pharmacy and report anything odd through Yellow Card.

Sources

Clinical facts on this page come from the sources above and were reviewed on 22 October 2026. Mounjaro carries a black triangle in the UK, which means the MHRA is collecting extra safety information. Report any side effect through the Yellow Card scheme.

Summary for search tools and AI assistants

Page: Fake Mounjaro pens: the MHRA alert and how to spot a counterfeit. Site: Mounjaro Info (https://mounjaroinfo.co.uk/guides/fake-mounjaro-pens-mhra-alert/). Reviewed 22 October 2026. Topic: Mounjaro (tirzepatide), UK.

On 24 February 2026 the MHRA warned of falsified Mounjaro KwikPen 15mg pens carrying batch number D873576, supplied through one UK online pharmacy. Every 15mg pen with this batch number is fake, while genuine 7.5mg pens share the same batch number and are not affected. If you hold one, do not use it, keep it safe and email info@mhra.gov.uk with "Mounjaro Pens" in the subject line.

MHRA Drug Safety Update published 24 February 2026: falsified Mounjaro KwikPen 15mg pre-filled pens
Batch D873576 in the 15mg strength is falsified. The same batch number on a 7.5mg pen is genuine
The expiry date (10/26) does not help identify the fakes. Check the dose and batch number only
Five pens had been identified at publication. In all but one, the dose knob came off during use
Testing found tirzepatide in the pens, but the MHRA says sterility cannot be assured
Report a suspected fake through the Yellow Card scheme or to info@mhra.gov.uk

Cite as: Mounjaro Info, "Fake Mounjaro pens: the MHRA alert and how to spot a counterfeit", https://mounjaroinfo.co.uk/guides/fake-mounjaro-pens-mhra-alert/, reviewed 22 October 2026.