Which weight loss injection holds the number 1 spot — and what does that actually mean?

Step 1: understand what the trial data actually shows

When people search for the 'number one' weight loss injection, they are usually asking which one produces the most weight loss, and our page on the number one weight loss injection breaks down how that question is answered by the clinical evidence. The largest head-to-head evidence comes from the SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, which compared tirzepatide directly with semaglutide 2.4mg over 72 weeks in adults with obesity and without diabetes. Tirzepatide produced the greater average weight reduction. Before SURMOUNT-5, the comparison was indirect: SURMOUNT-1 (tirzepatide, ~2,500 participants) reported roughly 20–21% average loss at 15mg, while the STEP 1 trial (semaglutide 2.4mg) reported around 15%.

The picture shifted again when the MHRA approved the Wegovy 7.2mg single-dose pen in April 2026. Trials of that higher dose reported approximately 20.7% average loss over 72 weeks, which puts it in similar territory to tirzepatide. In practical terms, the gap between the two medicines is narrower now than it was two years ago. Our overview of weight loss injections sets out both medicines side by side with their licensed uses and eligibility thresholds. Which one is better for you is genuinely a clinical question, the answer depends on your health history, other medications, and how you tolerate each medicine.

One more thing worth knowing: trial averages mask a wide range of individual responses. Some participants lose considerably more than the average; others less. A single headline figure does not tell you what you personally will experience.

Step 2: know the eligibility criteria before you assume either is available to you

Both Mounjaro and Wegovy are licensed 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, high cholesterol, type 2 diabetes, prediabetes, or obstructive sleep apnoea. Lower BMI thresholds can apply for some ethnic backgrounds under UK clinical guidance. BMI alone is not the whole picture, a prescriber looks at your full health history, current medicines, and any contraindications before making a clinical decision.

A question our prescribers hear most weeks is whether someone with a BMI just under 30, and no listed comorbidity, can access either injection. For private prescribing, the licensed thresholds above apply. There is no shortcut around them, and any service offering these medicines without a proper clinical assessment is not operating within the law. You can read more about how eligibility works across different weight loss injections to see which conditions qualify and what the assessment process involves.

Pregnancy, breastfeeding, and planning to conceive are all situations where neither medicine is recommended. The same applies to anyone under 18. People with a history of medullary thyroid carcinoma, MEN2, or certain gastrointestinal conditions will need to discuss their suitability carefully with a prescriber. These are not bureaucratic hurdles; they are safety-critical checks.

Step 3: understand the process of starting, and what the first weeks look like

Both medicines start at a low tolerability dose rather than the therapeutic dose seen in trials. Tirzepatide begins at 2.5mg; the first pen's job is to let your system adjust, reducing the likelihood of significant nausea during the weeks that follow. Dose increases are managed by your prescriber, typically in four-week steps, up to the highest dose you tolerate well. Wegovy follows a similar principle: the starting dose is 0.25mg, titrating up through several steps before reaching the maintenance dose.

The most common side effects during this process are gastrointestinal: nausea, changes in bowel habits, indigestion, and reduced appetite. These are generally most noticeable after starting or after a dose increase, and they tend to settle. If you are researching a weight loss injection r starting with that letter and want to understand how Rybelsus or related options compare on side effects and titration, our dedicated page covers the specifics. Injections are given once a week into the abdomen, thigh, or upper arm, rotating the site each time. If you are at the start of your research into how injections are administered day to day, this guide to Mounjaro injections covers the practical steps.

Safe disposal matters too. Every used pen is clinical waste. A sharps container keeps used pens safely contained until your local council or pharmacy sharps scheme collects them.

Accessing a licensed injection through a regulated pharmacy

Mounjaro and Wegovy are prescription-only medicines. In the UK that means the only lawful route to either is a prescription issued by a registered prescriber following a genuine clinical assessment. On the NHS, both medicines are available but access is phased and the waiting lists for Wegovy through specialist services can be long. Mounjaro is rolling out through GP practices from 2025 onwards, but criteria are strict and availability varies by area.

Private clinics and GPhC-registered online pharmacies offer an alternative for people who do not meet NHS thresholds or would prefer not to wait. If you are comparing costs across private providers, the context on what weight loss injection pricing actually includes is worth reading before making a decision, headline prices do not always reflect whether a prescription, clinical review, or aftercare is included. The MHRA has warned publicly about counterfeit pens sold online, some containing insulin rather than the labelled medicine; you can verify any online pharmacy on the GPhC register before using it. Report any suspect sellers via the MHRA Yellow Card scheme.

When you are ready to explore whether either medicine is clinically suitable for you, check your eligibility with our prescribers, consultation is free, and every submission is reviewed by a GPhC-registered Independent Prescriber the same day it arrives.