Which weight loss injection works best — and how do you decide?

The biggest misconception: 'best injection' implies one clear winner for everyone

It's a reasonable thing to google. But the framing itself carries a hidden assumption, that weight loss injections can be ranked like broadband deals. They can't. What clinical trials measure is average weight reduction across a large group of participants. Your result sits inside a distribution, not at its mean.

That said, the head-to-head data is real and worth understanding. In the SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, tirzepatide produced greater average weight loss than semaglutide 2.4mg over 72 weeks in adults with obesity and no diabetes. Tirzepatide's earlier SURMOUNT-1 trial reported an average reduction of around 20–21% of body weight at the 15mg dose. Wegovy at 2.4mg showed around 15% in its STEP 1 trial. The newer 7.2mg Wegovy dose, approved by the MHRA in January 2026, has narrowed that gap, reporting around 20.7% average loss over 72 weeks.

So tirzepatide still holds an advantage in the data. But NICE, in its appraisal of tirzepatide (TA1026), explicitly notes that indirect comparisons carry uncertainty. Trial populations differ. So does your physiology. A prescriber who knows your medical history is better placed to read these numbers than a search engine.

How the two medicines work differently, and why that gap in results may exist

Wegovy works on a single receptor: GLP-1. This pathway slows how quickly food leaves your stomach, damps appetite signals in the brain, and prompts the pancreas to release more insulin after eating. It is well-studied, effective, and has been in use long enough for a substantial real-world safety record to build up.

Mounjaro does all of that and adds a second pathway. It activates both the GLP-1 receptor and the GIP receptor, making it the only dual-agonist weight management medicine licensed in the UK. The combined effect on appetite and food reward appears to produce stronger suppression for many people, which may partly explain the larger average losses seen in trials.

Neither mechanism is 'better' in an absolute sense. For someone whose main challenge is appetite between meals, the extra GIP action may help considerably. For someone whose GI system reacts poorly to strong appetite suppression, a slower titration with either medicine (or a different approach altogether) may be the clinically wiser path. The full picture of how these injections work is worth reading before your consultation.

One practical habit worth building before you book: check your BMI and note any weight-related conditions you've been diagnosed with (high blood pressure, type 2 diabetes, sleep apnoea, high cholesterol). That information sits at the centre of every eligibility conversation and takes under a minute to gather.

What trial averages cannot settle: tolerability, personal history and clinical suitability

Both medicines share a broadly similar side-effect profile. Nausea tops the list, along with loose stools, constipation, indigestion and fatigue, especially in the weeks after a dose increase. For most people these settle, but they are real, and they influence whether someone stays on a medicine long enough to see results. A treatment that produces 21% average loss but that you stop at week eight because of persistent nausea has not worked better for you.

There are also clinical contraindications to work through. A history of pancreatitis, certain thyroid conditions, some medicines that interact with delayed gastric emptying, these shape which medicine, if either, is appropriate. Pregnancy, breastfeeding and trying to conceive: neither is recommended in those circumstances. Under-18s are outside the licensed population for both.

This is not a list of reasons to feel discouraged. It is the reason clinical assessment exists. Our prescribers at nume review your answers personally (not by algorithm) before any treatment decision is made. If you'd like to understand costs before going further, our treatment cost guide sets out what to expect from private providers in the UK.

Getting from 'which is best' to 'which is right for me'

The question most people mean to ask is not which injection wins a trial, but which one a prescriber would recommend given everything about them specifically. That shifts the frame from comparison shopping to clinical conversation, which is the right frame.

The NHS route to both medicines exists (tirzepatide under NICE TA1026 and Wegovy via specialist weight management services) but access criteria are strict and phased. Many people who are eligible in principle face waiting lists that stretch months. A regulated private service offers an alternative for those who meet the clinical criteria but don't want to wait, or who fall outside the NHS thresholds. Weight loss injections as a category are worth understanding in full before choosing a route.

A useful place to start is our page on comparing the licensed options in more detail, or if speed of response matters to you, the evidence on how quickly each medicine takes effect. When you're ready to talk through your own situation, check your eligibility with our prescribers, it's free, there's no obligation, and you'll have a real clinical answer rather than a search result.