The Synedica tirzepatide pen: what UK patients need to know

Step 1, Confirm you have the right, licensed pen

Before anything else, check the packaging. A legitimate UK pen will be labelled Mounjaro and carry Eli Lilly's name; the carton will show a batch number, expiry date and the GPhC-registered pharmacy that dispensed it. If your pen is labelled Synedica, or any name you cannot trace to an MHRA-authorised UK product, do not use it. The MHRA has warned repeatedly that unlicensed tirzepatide pens have entered the UK market through unofficial routes, and some have been found to contain the wrong substance entirely. You can verify the legitimacy of an online pharmacy dispensing your medicine on the GPhC pharmacy register.

If you are unsure about the source of a pen you already hold, report it to the MHRA's Yellow Card scheme, which handles reports of suspected counterfeit or substandard medicines. That is not a bureaucratic step, it is how these products get taken off the market.

Step 2, Prepare the pen properly before each dose

Mounjaro is stored in the fridge at 2–8°C. Take the pen out and let it sit at room temperature for around 30 minutes before you inject; cold solution tends to sting more. Check the pen's viewing window, the liquid should be clear to slightly yellow and free of visible particles. A pen that looks cloudy, discoloured or contains floating matter should not be used; contact your pharmacy or prescriber.

Attach a fresh needle to the pen for every injection. Reusing a needle blunts the tip, increases the chance of discomfort and raises the risk of infection at the injection site. If you need a reliable supply of compatible needles, replacement needle packs are available separately. Wipe the skin at your chosen site with an alcohol swab and let it dry fully before you inject, damp skin can sting and slows drying of the site.

Our detailed guide to using a tirzepatide pen walks through pen preparation in full, including what the clicking mechanism sounds and feels like when it is working correctly.

Step 3, Inject, confirm delivery, and dispose safely

Choose a site: the soft skin of the abdomen (at least 5 cm from the navel), the front of the thigh, or the upper arm. Rotate your location each week so the same patch of skin gets a break. Pinch the skin lightly if it feels more comfortable, press the pen firmly against your skin, and press the button. Hold it in place until the pen's indicator confirms the full dose has been delivered, the KwikPen has a built-in end-of-dose window for this. If you want a visual walkthrough of exactly what to do at this stage, our step-by-step guide on how to inject tirzepatide covers positioning, technique and confirming delivery in detail. Do not rub the skin after withdrawing the needle.

Remove the used needle immediately after the injection and place it in an approved sharps container, never into a household bin. A one-litre sharps container is the right size for home use with a weekly injectable. Return full containers to a pharmacy or council disposal point; local councils operate sharps collection schemes in most areas of England.

For a more detailed walkthrough of the injection itself, our tirzepatide injection guide covers technique, common mistakes and what to do if you are not sure the dose delivered fully.

Side effects worth knowing about, and what to act on quickly

The most common side effects of tirzepatide are gastrointestinal: nausea, loose stools, constipation, indigestion and burping, especially in the first few weeks or after a dose increase. For most people these settle as the body adjusts. Eating smaller portions, avoiding very fatty or rich meals, and staying well hydrated all help. If vomiting or diarrhoea is severe, keep up fluids carefully, prolonged GI illness on any GLP-1 medicine can affect kidney function.

One side effect deserves specific mention. The MHRA issued a Drug Safety Update in January 2026 highlighting acute pancreatitis as a known, though infrequent, risk with GLP-1 medicines. The key symptom to act on is severe stomach pain that spreads towards the back, particularly if it does not ease and comes with vomiting. That warrants urgent medical attention, not a wait-and-see approach. The NHS tirzepatide medicines page lists the full side-effect profile and explains which symptoms need same-day medical contact.

We know that starting a new injectable medicine can feel daunting, and worrying about whether you are doing it right is entirely normal. If anything feels off during injection or in the hours afterwards, our aftercare team is available seven days a week. You can also browse common patient questions for quick reassurance on injection technique and reactions.

For background on Mounjaro as a treatment (how it works, what results the clinical trials reported, and the cost picture in the UK since Eli Lilly's 2025 price change) the Mounjaro overview covers all of that in one place. If you are weighing up whether to start treatment, and would like to understand more about the tirzepatide pen you will be using, check your eligibility with our prescribers, consultations are free and reviewed the same day by a named clinician.