The Mounjaro Checklist: What to Verify Before Your First Pen

The biggest myth: that checking the pen is only for people who bought from a dodgy source

Most people who end up with a counterfeit or degraded Mounjaro pen did not buy it from an obviously suspicious seller. They bought it from a site that looked professional, had a working checkout, and even issued something resembling a prescription. The MHRA's enforcement work through 2025 (which led to the seizure of roughly 20 million doses of illegally traded weight-loss medicines worth around £45 million) showed that fake pens have circulated through channels that appeared legitimate on the surface.

So the first misconception this checklist corrects is that verification is someone else's problem. It is yours, whoever supplied the pen. The quickest check you can run is on the carton itself: every genuine Mounjaro pen carries a serial number that can be looked up on our Mounjaro serial number check page, which explains the process step by step. That takes under a minute and costs nothing. Counterfeit pens have been found to contain entirely different substances, including insulin in doses that caused serious harm. Authentic packaging, an intact carton seal, and a verifiable serial number are not optional extras.

A GPhC-registered online pharmacy sourcing through the licensed UK supply chain (which you can verify at the GPhC register) is the foundation of a safe supply, but the physical pen check remains worthwhile regardless.

Checklist items that relate to your clinical eligibility

The licensed criteria for Mounjaro as a weight-management medicine are set out in the product's Summary of Product Characteristics and discussed in NICE's appraisal of tirzepatide (TA1026). For private prescription purposes, the relevant thresholds are a BMI of 30 or above, or 27 or above if you have at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol, or obstructive sleep apnoea.

Before your consultation, it helps to know your current weight and height accurately, and to have a recent blood pressure reading if you have one. A prescriber needs to assess whether Mounjaro is appropriate for your specific health picture, not just whether a number clears a threshold. Conditions that require careful prescriber discussion include a personal or family history of medullary thyroid carcinoma, MEN2, certain gastrointestinal conditions, and a previous history of pancreatitis. You should also flag any other medicines you take: oral contraceptives in particular require an additional non-oral method of contraception for the first four weeks of Mounjaro treatment and for four weeks after each dose increase, because absorption may be affected. The NHS tirzepatide information page covers these interactions clearly.

If you are pregnant, breastfeeding, or planning to conceive, Mounjaro is not recommended. The same applies for anyone under 18. These are absolute points, not clinical grey areas.

Checklist items for your home setup before the pen arrives

This section gets less attention than eligibility, but it affects how your first few weeks go. Mounjaro pens must be stored in a refrigerator at 2–8°C. The exact window for keeping a pen at room temperature before it must be discarded is specified in the Patient Information Leaflet that comes with your pen, do read it, because it varies. A fridge shelf that keeps a consistent temperature (not the door, which fluctuates) is what you want. The pen should be taken out a short while before injecting to take the edge off the cold, though again, the leaflet is the authority on timing.

You will also need a sharps disposal bin. Used pens cannot go into household waste. If you have not sorted one yet, our Mounjaro shopping list page covers exactly this kind of practical preparation. Injection sites rotate between the abdomen, thigh, and upper arm; deciding in advance where you plan to start and keeping a simple mental note of where you injected last makes that rotation easy to maintain.

For anyone thinking about what treatment costs before the first consultation, our Mounjaro cost page gives factual context on UK private pricing.

Confirming your source: what a legitimate supply route looks like

Mounjaro sold legally in the UK comes via a licensed pharmacy following a genuine clinical assessment. Any offer that skips that step is unlicensed supply, regardless of how the website is presented. Red flags include sellers who offer pens without any prescriber involvement, prices that sit noticeably below the private market range, and (particularly) discount codes for prescription medicines from sellers you cannot verify. The MHRA is clear: report suspected illegal sellers through the Yellow Card scheme at yellowcard.mhra.gov.uk.

If you are considering treatment and want to understand what the other name for tirzepatide is before your first consultation, our tirzepatide overview gives a full clinical background. When you are ready, a free consultation with one of our prescribers is the straightforward next step, no waiting list, and a real clinician reviews your answers the same day, and you can also browse our Chemist for You Mounjaro service to see how we supply it.