Do weight loss injections actually work — and what does the evidence say?

You've read the headlines, here's what the numbers actually mean

Picture the person who first hears about these injections from a headline: "up to 22% weight loss". Scepticism is reasonable. That figure comes from SURMOUNT-1, a 72-week randomised trial involving 2,539 adults with obesity, published in the New England Journal of Medicine. Participants received tirzepatide at 5, 10 or 15mg, alongside structured lifestyle support. The 15mg group lost around 20–21% of body weight on average. Some individuals in specific analyses reached 22.5%, that is the figure that surfaced in coverage, and it is real, but it is the upper end, not the typical experience at every dose.

Semaglutide (Wegovy) has its own large dataset. The STEP 1 trial, 68 weeks, found an average loss of roughly 15% at the 2.4mg maintenance dose. A newer, higher 7.2mg dose approved by the MHRA in early 2026 is producing results closer to tirzepatide in trials. So yes, these figures are genuine, peer-reviewed and reproducible. The critical word in both trials is alongside: participants followed diet and activity programmes. The injection does not override physics; it changes the biological signals that make sticking to a deficit so difficult for many people.

If you want to understand exactly how the mechanism of these injections works, that detail is worth reading before you start. The short version is that these medicines mimic gut hormones that slow gastric emptying, increase satiety and, in tirzepatide's case, activate a second receptor pathway that also improves insulin sensitivity. Hunger genuinely reduces. That is why the trial results look different from previous weight-management medicines.

The honest limits, what the evidence also shows

The same trials that show large average losses also show that results vary considerably between individuals. Some people lose far more than the average; others lose meaningfully less. Stopping treatment tends to reverse much of the weight loss over time, a fact that is well documented and that prescribers should discuss with you openly. These are not short courses for most people who benefit; they are ongoing treatments with regular clinical review.

Side effects are common, particularly at the start or after a dose increase. Nausea, loose stools, constipation and indigestion are the most frequently reported; they are generally mild to moderate and tend to ease within days to a couple of weeks as the body adjusts. Serious side effects are less common but do exist. In January 2026 the MHRA issued a Drug Safety Update specifically highlighting acute pancreatitis as an infrequent but potentially serious risk: if you develop severe, persistent stomach pain that spreads to your back, seek medical help promptly rather than waiting to see whether it passes.

There are also populations for whom these medicines are not suitable: pregnancy, breastfeeding, or planning to conceive rules them out, as does a history of certain thyroid or pancreatic conditions. Under-18s are not a licensed population. A prescriber assessing your full picture (not just your BMI) is the step that makes treatment both safe and appropriate for you specifically. Whether fat distribution affects how you respond is a question worth exploring separately; the short answer is that visceral fat does reduce, but the injection does not target abdominal fat in isolation.

NHS access, private access, and what actually arrives

NHS access to these medicines exists but is being rolled out in carefully phased stages, with strict BMI and comorbidity thresholds that most people reading this will not currently meet. Tirzepatide on the NHS requires, at present, a BMI of 40 or above combined with four or more specific weight-related conditions. The NHS England guidance on weight management injections sets out the full pathway and explains what wraparound support must accompany any NHS prescription.

For people who do not meet those thresholds, or who do not want to wait, a private prescription through a GPhC-registered pharmacy is the legal alternative. The process matters: a legitimate provider requires a clinical consultation, identity verification, and a prescriber to review your information before any medicine is issued. At nume, that review is carried out the same day by a GPhC-registered Independent Prescriber. Once approved, the pen is dispatched on the same day (for orders placed before 12pm, Monday to Friday) and arrives by tracked DPD delivery the next working day, a small, plain box, nothing on the outside to indicate what is inside.

The broader landscape of weight management options is worth understanding too, because injections are not the right route for everyone. For some people the clinical assessment will point to a different approach, or to injections alongside other support. That is precisely what the consultation is for. If you are curious about what differentiates one injection from another, comparing the two licensed options in more detail helps frame that conversation.

One contextual note on cost: private treatment for these medicines is not cheap, and the treatment pricing page is the right place to see current figures rather than approximations in an article. What that price should always include (legally and practically) is the consultation, the prescription, and ongoing clinical support. At nume, delivery and aftercare are included too, with no subscription and no automatic renewals.