The Top Weight Loss Injections Reviewed: What the Evidence Actually Shows

How the clinical evidence compares for both licensed injections

The strongest data we have comes from two landmark programmes. STEP 1, published in the New England Journal of Medicine, followed adults with obesity over 68 weeks on semaglutide 2.4mg. Average weight loss was around 15% of starting body weight. That was a genuinely significant result, enough for NICE to recommend Wegovy through specialist weight management services.

Then came SURMOUNT-1, which tested tirzepatide across 72 weeks in 2,539 adults with obesity. At the highest dose (15mg), average weight loss reached around 20–21%. SURMOUNT-5 went further still: in a direct head-to-head against semaglutide 2.4mg over the same period, tirzepatide produced greater average loss. NICE's appraisal of tirzepatide (TA1026), published in December 2024, concluded the evidence was strong enough to recommend it for eligible adults on the NHS. Both medicines start at a low tolerability dose and increase gradually, the goal is to let your system adjust before the dose does any real work.

A newer option is the Wegovy 7.2mg pen, approved by the MHRA in April 2026. Early data suggests average weight loss of around 20.7% at that higher dose, narrowing the gap with tirzepatide's top-end results. The titration schedule is the same: you start low and work up, guided by a prescriber.

None of these figures are guarantees. They are averages from controlled trials alongside lifestyle changes. Real-world results depend on adherence, diet, activity and individual biology.

What the injections actually do, and why two receptors matter

Both medicines slow how quickly food leaves the stomach, reduce appetite and help regulate blood sugar. The difference is where they act. Semaglutide activates GLP-1 receptors, hormone pathways involved in fullness and insulin release. Tirzepatide activates both GLP-1 and GIP receptors. GIP is a second gut hormone that also plays a role in energy balance; hitting both appears to produce a stronger appetite-suppressing effect in most people, which is the most likely explanation for the larger average weight losses seen in trials.

Neither is a passive treatment. Both work best alongside a reduced-calorie diet and regular physical activity, the licence and every clinical guideline are clear on that. The injection handles a significant part of the biology; the lifestyle piece handles the rest. If you want a fuller picture of how these medicines fit into a broader weight management plan, the weight loss treatments overview sets that out clearly.

Side effects are mostly gastrointestinal: nausea, loose stools, constipation, reflux. They tend to be most noticeable after starting or after a dose increase, and for most people they settle within a couple of weeks. Rarely, acute pancreatitis can occur, seek urgent medical attention for severe, persistent stomach pain that radiates to the back. The MHRA highlighted this risk in a January 2026 Drug Safety Update for GLP-1 medicines, and it is worth knowing before you start.

Who the evidence applies to, and who these injections are licensed for

The trial populations tell you a lot about who the medicines are designed for. SURMOUNT-1 and STEP 1 enrolled adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition such as high blood pressure, high cholesterol, type 2 diabetes or obstructive sleep apnoea. The UK licences reflect that: both medicines are approved for adults in those BMI ranges, alongside a qualifying condition at the lower threshold.

NICE's NHS recommendations are narrower than the licence. Tirzepatide on the NHS currently requires a BMI of 35 or above plus at least one of a defined list of conditions; semaglutide via NHS specialist services carries a two-year maximum treatment period and a multidisciplinary support requirement. BMI thresholds are adjusted downward by 2.5 kg/m² for some ethnic backgrounds under UK guidance. For anyone who does not meet those NHS criteria (or who does not want to wait) a private prescription through a regulated pharmacy is the legal alternative, subject to the same clinical assessment.

If you are weighing up the two medicines, the top-rated weight loss injections comparison looks at both options side by side. And if you want to understand how ordering actually works through a registered online pharmacy, where to buy weight loss injections in the UK covers the process and what to check.

How to pursue treatment safely and what makes a legitimate route

Both Mounjaro and Wegovy are prescription-only. That means the only legal route is a consultation with a registered prescriber who reviews your health history, weight, medicines and any contraindications before deciding whether treatment is appropriate. Any seller offering these injections without that step is operating outside the law, and in practice, often outside the legitimate supply chain entirely. The MHRA has seized large volumes of counterfeit weight loss pens in recent years, some containing dangerous substitutes. If you are researching your options and want to know which website for weight loss injections is worth trusting, checking any online pharmacy on the GPhC's pharmacy register before handing over any personal or payment details is a sensible first step.

At nume, every consultation is read by a GPhC-registered Independent Prescriber, a real clinician, the same day. Treatment ordered before noon on a working day is dispatched the same day once approved, arriving free the next working day via DPD in plain packaging. There are no subscriptions. Every repeat order goes through the same clinical review from scratch. One note worth knowing if you are thinking about timing: orders placed on a Monday morning (or just after payday) tend to be the smoothest, since you have the full working week ahead for delivery if anything needs a follow-up.

For anyone ready to find out which of the top 5 weight loss injections might suit their situation, start your free consultation and a prescriber will take it from there. You can also read about weight loss injections in general or browse the FAQs if you have specific questions before you begin, and if you want a wider view, a guide covering the top 10 weight loss injections is available to help you compare the full landscape.