The New Weight Loss Injection: What It Is and How It Works

You've read the headlines, here's what's actually in the pen

Picture this: you've spent twenty minutes scrolling through news stories about the "new weight loss jab" and you still aren't sure whether the medicine in question is safe, what it's called, or whether it's even legal to get hold of in the UK. That's the situation most people are in when they first search this topic.

The medicines driving those headlines are tirzepatide, sold as Mounjaro, and semaglutide, sold as Wegovy. Both are injectable medicines given once a week via a pre-filled pen. Tirzepatide is made by Eli Lilly and is classed as a dual GIP and GLP-1 receptor agonist, meaning it activates two gut-hormone pathways involved in regulating appetite and blood sugar. Semaglutide, made by Novo Nordisk, targets the GLP-1 pathway only. You can read the patient-level overview of both on the NHS tirzepatide page.

In practical terms, both medicines work by slowing how quickly your stomach empties after eating, increasing feelings of fullness, and reducing appetite signals in the brain. Treatment starts at a low dose to let your body adjust, then is titrated upward by a prescriber over several weeks. Neither is a quick fix; both are intended to be used alongside a reduced-calorie diet and increased physical activity. The pen itself sits in the fridge door, taken out once a week on the same day each time, a routine that becomes habit quickly for most people.

If you want to understand how weight loss injections work in more detail, that page covers the mechanism alongside what to expect from each pen format.

What the clinical trials actually found

The results from the trials for these medicines are larger than most new prescription treatments produce. In SURMOUNT-1, a phase 3 trial published in the New England Journal of Medicine involving 2,539 adults with obesity, participants taking tirzepatide 15mg lost an average of around 20 to 21% of their body weight over 72 weeks. Head-to-head in the SURMOUNT-5 trial, tirzepatide produced greater average weight reduction than semaglutide 2.4mg over the same period.

Semaglutide's STEP 1 trial reported around 15% average weight loss at the 2.4mg weekly dose over 68 weeks. The newer 7.2mg semaglutide dose, approved by the MHRA in early 2026, showed around 20.7% average weight loss in trials, bringing it closer to tirzepatide's results. NICE has appraised both medicines: TA1026 recommends tirzepatide for adults with a BMI of 35 or above and at least one weight-related condition, and TA875 recommends semaglutide within specialist weight management services.

These figures are averages from clinical trial populations; individual results depend on dose, adherence, diet, activity and other health factors. No prescriber can guarantee any particular outcome for any individual, and a prescriber won't promise one either. What the evidence does show is that, for many people, these medicines produce meaningful, clinically significant weight reduction when used as licensed.

For a comparison of how the two medicines stack up, this page on the new weight loss drug injection sets them alongside each other with the trial data.

Side effects: what's common, what's serious, and what to watch for

The side-effect profile for both medicines is led by the gut. Nausea is the most frequently reported effect, particularly in the first few weeks after starting or after a dose increase. Vomiting, diarrhoea, constipation, bloating, indigestion and fatigue also appear in the trial data. For most people these effects are mild to moderate and ease within a week or two. Starting at the lowest dose and titrating gradually is specifically designed to reduce how disruptive this period feels, it's why the starter dose exists.

More serious reactions are uncommon but warrant prompt attention. Severe, persistent stomach pain that reaches through to your back (with or without vomiting) can be a sign of acute pancreatitis, and you should seek urgent medical help if this happens. The MHRA issued a Drug Safety Update in January 2026 specifically highlighting pancreatitis as a known but infrequent risk with GLP-1 medicines. Gallbladder symptoms, signs of a serious allergic reaction, and significant dehydration following severe vomiting or diarrhoea also require prompt medical assessment.

Full safety information, including contraindications, is in the Patient Information Leaflet supplied with your medicine, and you can report any suspected side effects at the MHRA's Yellow Card scheme. If you have specific concerns about safety before committing to treatment, this page addresses whether the new weight loss injection is safe in more depth.

Getting access: NHS, private, and what a legitimate route looks like

These are prescription-only medicines. The only legal way to obtain them in the UK is through a prescription issued by a qualified prescriber following a clinical assessment. That assessment covers your BMI, your medical history, any medicines you currently take, and whether there are reasons the treatment would not be suitable for you, certain thyroid conditions, a history of pancreatitis, pregnancy, and a number of other factors are among the contraindications a prescriber checks.

On the NHS, access to tirzepatide is currently phased: the NICE TA1026 rollout began in June 2025 for people with a BMI of 40 or above and four or more specified weight-related conditions, with eligibility broadening in stages through to 2027. Wegovy is available through specialist weight management services, though waiting lists are often long. For context on what private treatment costs, this breakdown of weight loss injection costs sets out what typical private pricing includes and what to watch for.

If you're exploring whether private treatment is the right route, a consultation with a GPhC-registered prescriber is where that decision gets made properly. Our guide to getting the new weight loss injection covers eligibility criteria step by step. And if you'd like to understand what a responsible online pharmacy looks like, you can verify any pharmacy on the GPhC register, our registration number is 9012878. When you're ready to speak to a prescriber, you can start your free consultation with our clinical team, and if you are based in Scotland, our page on weight loss injections in Glasgow covers what local access looks like in practice.