Mounjaro®
Starting from £179.99/mo
Start journey Learn moreHow these injections actually work, who can use them, and what to expect week to week
Picture this: you've heard about weight-loss jabs but aren't sure what they actually do
You've probably seen the headlines. Someone you know has lost two stone. The waiting list for the NHS version stretches to years. And the online market is a maze of unfamiliar names and prices that seem too good to be true. Before anything else, it helps to understand what these injections are actually doing inside your body, because once you do, a lot of the noise falls away.
Both Mounjaro and Wegovy mimic hormones your gut releases after eating. GLP-1 (glucagon-like peptide-1) is one of them; it signals fullness to the brain, slows the rate at which your stomach empties, and reduces appetite. Tirzepatide also activates a second pathway (GIP (glucose-dependent insulinotropic polypeptide)) making it the only dual-agonist weight-loss medicine licensed in the UK. The practical result, for most people, is that hunger quietens noticeably. Meals feel satisfying sooner. The drive to snack between them fades. None of this replaces sensible eating or activity (both licences require a reduced-calorie diet and increased movement alongside the medicine) but for many people, the biology that made those things feel impossible shifts.
You can find a broader look at the weight-loss injection landscape on our treatment overview, or explore the full breakdown of which injection helps with weight loss if you want to compare the two medicines side by side before reading further.
Who these injections are licensed for, and where the lines are drawn
The licence for both medicines in the UK covers adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition. That list of conditions includes type 2 diabetes, high blood pressure, raised cholesterol, obstructive sleep apnoea, and cardiovascular disease, among others. For some ethnic backgrounds (including South Asian, Chinese, Middle Eastern, Black African and African-Caribbean) UK guidance applies a BMI threshold 2.5 kg/m² lower, recognising that health risks cluster at a lower BMI in those groups.
Neither medicine is licensed for people under 18. Both are not recommended during pregnancy, while breastfeeding, or when actively trying to conceive. Certain medical histories (including previous pancreatitis, medullary thyroid carcinoma, or a condition called MEN2) require careful prescriber discussion before treatment can be considered. BMI alone is never the whole picture; a prescriber weighs the full clinical context. The eligibility detail for these injections covers the clinical criteria more fully if you want to check before booking.
People sometimes ask whether these injections help specifically with PCOS-related weight gain. The research is still developing, but the PCOS question is one we address separately, the answer is more nuanced than a simple yes or no.
What using one of these pens actually looks like week to week
This is where the theory meets Monday morning. Both medicines come as pre-filled, multi-dose pens. You inject once a week, rotating between sites, typically the stomach, outer thigh or upper arm. The needle is fine; most people find the injection itself takes seconds and causes little more than mild stinging, if that.
Tirzepatide starts at 2.5mg. That first pen's job is to let your system adjust, not to deliver the full therapeutic effect straight away; the dose steps up every four weeks or so under prescriber guidance. Semaglutide follows a similar escalation from 0.25mg up to 2.4mg (or 7.2mg under the newer pen, approved by the MHRA in April 2026). Both pens need to be kept refrigerated, a lot of people slot theirs in the fridge door, which is fine as long as it stays between 2°C and 8°C. For the exact storage window and advice on room-temperature handling, the eMC SmPC for your specific pen is the source to trust, not general guides like this one.
Side effects are mostly gastrointestinal: nausea, loose stools and constipation are the ones people most commonly report, especially in the early weeks or after a dose increase. They usually settle. If you experience severe, persistent stomach pain that radiates to your back, get medical help promptly, that pattern needs clinical assessment. The MHRA issued specific guidance on pancreatitis as a known, infrequent but serious risk with GLP-1 medicines, and it is worth being aware of.
If you need a sharps disposal solution at home, a 1-litre sharps container makes disposal straightforward and safe. Our main weight-loss injections page covers the practicalities of starting treatment in more depth, including what to bring to your consultation.
NHS, private, and what to watch out for online
On the NHS, access to these medicines is tightly rationed. Mounjaro is currently available to people with a BMI of 40 or above alongside four or more specific weight-related conditions; the criteria will broaden in phases through 2027. Wegovy is available via specialist weight management services under NICE guidance, typically with long waiting lists. For most people asking about these injections today, the realistic route is a private prescription from a regulated online pharmacy.
That's where the risks live too. The MHRA has warned repeatedly about counterfeit pens sold online, with large seizures of fake products, some containing insulin rather than the labelled medicine. The safest check is simple: any online pharmacy dispensing prescription medicines in the UK must be listed on the GPhC register. Sellers offering these medicines without a valid prescription, or at prices dramatically below the market, are a red flag worth taking seriously. You can also check where to buy weight-loss injections safely for a fuller look at what legitimate supply looks like. And if B12 injections have come up in your research as a weight-loss option, our page on whether B12 injections help with weight loss gives you the honest answer.
At nume, our GPhC-registered pharmacy (registration 9012878) sources medicines through the licensed UK supply chain. Every consultation is reviewed personally by a GPhC-registered Independent Prescriber. You can learn more about our clinical team, including our lead prescriber, on the about pages. If you'd like to check your suitability, the consultation is free. Speak to our prescribers and get a clear answer about whether one of these injections is right for you.
Looking to start your weight loss journey?
Take a quick eligibility quiz to explore your options and see how we can support you.