How effective is the weight loss injection, according to the evidence?

The SURMOUNT-1 and STEP 1 figures: what they measured, and what they mean

The two landmark trials are worth understanding on their own terms, not just as headline percentages. SURMOUNT-1 randomised 2,539 adults with obesity or overweight plus at least one weight-related condition over 72 weeks. At the 15 mg dose of tirzepatide, participants lost around 20–21% of their starting body weight on average — in some analyses, around 22.5%. That is published in the New England Journal of Medicine's SURMOUNT-1 report, and it is the figure NICE drew on when recommending tirzepatide in December 2024.

STEP 1 enrolled 1,961 adults over 68 weeks. Semaglutide 2.4 mg produced an average loss of around 15% of body weight, versus just under 3% with placebo. The full results are in the STEP 1 paper in the New England Journal of Medicine. Both trials required participants to follow lifestyle guidance alongside the medicine, which reflects how these treatments are licensed: as adjuncts to a reduced-calorie diet and more activity, not standalone interventions.

A few practical points get lost when people share these figures. Averages include participants who responded very strongly and some who responded modestly. Results build over months, not weeks. The starter dose exists to let your digestive system settle, not to produce rapid loss, the therapeutic effect grows as the dose increases under a prescriber's guidance. That process takes most people six months or more to reach the maintenance dose.

How tirzepatide and semaglutide compare head-to-head

For a long time, comparing these two medicines relied on cross-trial data, which has limitations. SURMOUNT-5 changed that: a 72-week open-label trial published in the New England Journal of Medicine in 2025 directly compared tirzepatide with semaglutide 2.4 mg in 751 adults with obesity and no diabetes. Tirzepatide produced greater average weight reduction. NICE's committee discussion in technology appraisal TA1026 also noted that indirect comparisons favour tirzepatide.

The reason is likely the mechanism. Semaglutide activates the GLP-1 receptor, which slows gastric emptying and increases fullness signals. Tirzepatide activates both GLP-1 and GIP receptors, a dual action that appears to produce a stronger overall effect on appetite and body weight. If you want to read more about which injection produces the strongest results, that comparison covers the evidence in more depth.

It is worth noting that a newer, higher dose of semaglutide (the 7.2 mg weekly dose, approved by the MHRA in April 2026) has trial data showing around 20.7% average weight loss over 72 weeks, which closes much of the gap. The clinical picture is still developing, and the best choice for any individual depends on their health history, tolerability, and what a prescriber judges appropriate.

What affects how much weight you actually lose

Trial averages describe a population, not a prediction. Several factors shape individual results. Adherence matters greatly: people who stay on the full titration schedule and maintain dietary and activity changes tend to see results closer to the upper end of trial ranges. Those who stop and restart, or who do not reach the maintenance dose, typically see less.

Tolerability plays a role too. The most common side effects are gastrointestinal (nausea, loose stools, indigestion, occasional vomiting) and they are most noticeable in the early weeks or after a dose increase. Most people find them manageable and transient. The slow titration schedule exists precisely to reduce their intensity. If side effects do feel unmanageable, the prescriber can slow the titration; that is a normal part of clinical oversight, not a treatment failure.

Starting weight, metabolic history, activity levels, sleep quality, and the specific medicine all contribute. Some people reach their goal weight before hitting the maximum dose. Others need the full schedule. There is genuine biological variation here, and an overview of how weight loss injections work covers the mechanisms behind that variation.

One practical note: timing your order matters more than people expect. Order by 12pm Monday to Friday once clinically approved, and treatment is dispatched the same day for next-working-day delivery, useful to know if you are planning around a payday, a Monday start, or a holiday when deliveries run differently. Details on how that works sit on our weight loss injections page.

Effectiveness in the context of safety and clinical oversight

Effectiveness and safety are not separate questions. These medicines are prescription-only precisely because the prescriber's role is to weigh both together. Anyone offering them without a clinical assessment is not offering a safer or cheaper equivalent; they are removing the safeguard that makes the treatment appropriate to use.

The MHRA monitors these medicines closely. On 29 January 2026, it issued a Drug Safety Update highlighting acute pancreatitis as a known, infrequent but potentially serious side effect across GLP-1 medicines. Severe, persistent stomach pain that radiates to the back requires urgent medical attention. That update reinforces rather than undermines the case for proper clinical oversight: a prescriber who knows your history can identify risk factors in advance and monitor you during treatment.

If you want to explore the practicalities of subcutaneous injection for weight loss, including technique and what to expect, that page covers the process in plain terms. For a broader comparison between injectable and oral options, the question of whether weight loss pills match injections for effectiveness is addressed separately.

Our clinical team at nume, sorry, at our clinical team page explains the prescriber-led model in more detail. A consultation is free, there are no subscriptions, and every repeat order is clinically reviewed. If you have read the evidence and want a prescriber to assess whether treatment is right for you, the straightforward next step is to speak to our prescribers via the weight loss treatments page.