The WAG Weight Loss Injection Explained

You've heard the name. Here's what's actually in the pen.

Picture a colleague who mentions she's been on the "WAG jab" for three months and has lost a stone and a half. She says a friend of a friend got it sorted online, no fuss. That single conversation now has you Googling at 11pm. The reality is both more straightforward and more clinically involved than the phrase suggests.

The injections behind the nickname are Mounjaro (tirzepatide, made by Eli Lilly) and Wegovy (semaglutide, made by Novo Nordisk). Mounjaro works on two gut-hormone pathways simultaneously, GIP and GLP-1, making it the only dual-agonist weight-loss medicine currently licensed in the UK. Wegovy acts on the GLP-1 pathway alone. Both slow the rate at which the stomach empties, signal the brain that food has already arrived, and progressively reduce how much you want to eat. Treatment starts at a low tolerability dose and is titrated upwards by a prescriber over several months, so the medicine your colleague is taking now is almost certainly not the dose she started on. You can read a broader overview of how licensed weight loss injections work and what separates the different options.

In the SURMOUNT-1 trial, participants on the highest tirzepatide dose lost around 20–21% of body weight on average over 72 weeks, according to data published in the New England Journal of Medicine. The STEP 1 trial for semaglutide 2.4mg showed approximately 15% average reduction over 68 weeks. Both figures are trial averages, and real-world results span a range. Neither is a guarantee.

What celebrity coverage leaves out about eligibility and safety

The tabloid version of these medicines skips the bit where a clinician decides whether they are appropriate for you. Under UK licensing, both Mounjaro and Wegovy are indicated for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. A BMI in range does not automatically mean the medicines are suitable. Contraindications, current medications, and medical history all feed into the prescriber's decision.

Side effects get even less column space in celebrity coverage. The most common are gastrointestinal: nausea, loose stools, constipation, indigestion and burping, which tend to be worst in the first week or two after a dose increase before settling. More serious, though infrequent, is acute pancreatitis. The NHS patient information for tirzepatide advises seeking urgent medical help for severe, persistent stomach pain that spreads to the back, with or without vomiting, as this can be a sign of pancreatitis. If you want a thorough breakdown of what to watch for, our page on weight loss injections covers the full side-effect picture with sources.

One practical note: these pens are refrigerated medicines. The habit that tends to work best is keeping your current pen in the fridge door where you'll see it, then setting a recurring phone reminder for your chosen injection day. The schedule is consistent enough that it quickly becomes routine, but the fridge reminder saves the moment of "wait, did I do it this week?"

How to access these medicines legally and safely in the UK

The only legal route to Mounjaro or Wegovy for weight management in the UK is a prescription issued following clinical assessment by a registered prescriber. That prescriber might be your GP, a hospital specialist, or an Independent Prescriber working at a regulated online service. There is no route that bypasses that step.

This matters because fake pens are a documented problem. The MHRA has warned repeatedly about counterfeit weight-loss injections sold via social media and unregistered websites, some of which have been found to contain insulin rather than the stated medicine. Checking that any online pharmacy you use is registered with the General Pharmaceutical Council takes about 30 seconds at the GPhC register. If a seller is offering these pens without requiring a prescription and a clinical consultation, that is the clearest possible warning sign.

Before starting, and at every repeat, a prescriber reviews whether treatment is still appropriate. That isn't bureaucracy for its own sake. Dose changes can affect how oral contraceptives are absorbed: women taking the pill alongside tirzepatide are advised to add a non-oral contraceptive method for the first four weeks of treatment and for four weeks after each dose increase. NHS England's guidance on weight loss injection interactions covers this and similar clinical considerations in detail. If you're weighing up which injection might be the better fit for your circumstances, the comparison of licensed weight loss injections available in the UK is a useful starting point, and if you'd like to understand exactly how tirzepatide is used in practice, our guide to the Mounjaro injection for weight loss walks through the dosing schedule, what to expect, and how it differs from other options. Our guide to weight loss injection prices sets out what a legitimate private prescription realistically costs.

At nume, every consultation is read and decided by a GPhC-registered Independent Prescriber, not automated software. If you want to understand the clinical team behind those decisions, you can read about our lead prescriber and how oversight works. When you're ready to take the step from reading about these medicines to finding out whether they're right for you, start your free consultation.