Peptide injections for weight loss: separating evidence from noise

How the evidence base for these peptide injections was built

The two licensed peptide injections for weight loss did not arrive quietly. Tirzepatide went through the SURMOUNT clinical programme, with SURMOUNT-1 alone randomising 2,539 adults with obesity but without diabetes and following them for 72 weeks. Published in the New England Journal of Medicine, the results showed average body-weight reductions of around 20–21% at the highest 15 mg dose. Semaglutide's phase-3 programme, the STEP trials, produced similar headline reading: STEP 1 (68 weeks, 2.4 mg) reported roughly 15% average weight reduction versus placebo.

These figures matter because they come from large, placebo-controlled trials with independent peer review, not marketing copy. NICE reviewed this evidence before recommending tirzepatide in December 2024 (technology appraisal TA1026) and semaglutide earlier under TA875. Both recommendations are grounded in the same body of data you can read directly. If you want a deeper look at how semaglutide sits within that evidence base, the weight loss injections overview covers both medicines side by side.

One checking habit worth doing before you commit to any provider: look them up on the GPhC pharmacy register. It takes under a minute and confirms the pharmacy is legally registered to dispense prescription medicines. Any seller that cannot be found there is not a legitimate route for a prescription-only peptide injection.

What these peptides are doing in your body

The word "peptide" simply means a short chain of amino acids, the same building blocks as proteins, just fewer of them. Your gut naturally produces peptide hormones after eating: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide) are two of them. They signal to the brain that food has arrived, slow the rate at which the stomach empties, and reduce appetite between meals. In people living with obesity, these signals are often blunted.

Semaglutide is a synthetic analogue of GLP-1 that mimics this hormone with a longer half-life, sustaining the signal for a full week from a single injection. Tirzepatide is structurally different: it acts on both the GLP-1 and GIP receptors simultaneously, making it the only dual-agonist weight-loss injection currently licensed in the UK. Whether that dual action explains the additional weight reduction seen in head-to-head data (including the SURMOUNT-5 trial published in 2025, which showed greater average loss with tirzepatide than with semaglutide 2.4 mg) is still an active area of research.

Neither medicine is a fat-burning compound in the popular sense. They work primarily by resetting the hunger and satiety signals that govern how much you eat. The detailed explainer on what these peptide injections are goes further into the receptor biology if you want it.

Who the UK licences cover, and who they do not

Both tirzepatide and semaglutide are licensed for adults aged 18 and over. The core eligibility threshold is a BMI of 30 or above, or a BMI of 27–29.9 alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, dyslipidaemia or obstructive sleep apnoea. Lower BMI thresholds can apply for some ethnic backgrounds under UK clinical guidance. A prescriber assesses the whole picture, not the BMI alone, past medical history, current medications and individual circumstances all feed into the decision.

There are people for whom these medicines are not appropriate: anyone who is pregnant, trying to conceive or breastfeeding; those under 18; and people with certain conditions including a personal or family history of medullary thyroid carcinoma or MEN2, or a history of pancreatitis. The eligibility guide covers the BMI and condition criteria in full. For older adults specifically, there is a separate page on safety considerations for seniors, because the prescriber's assessment weighs frailty, polypharmacy and individual health status carefully in that group.

It is also worth being direct about what is not licensed. Several peptides are sold online and marketed for weight loss (BPC-157, CJC-1295 and others) without a UK licence for this use and without the clinical trial evidence that tirzepatide and semaglutide carry. If you are weighing up which option is most appropriate for your circumstances, our guide to the best injectable peptide for weight loss compares the licensed medicines so you can assess the evidence for each. If you have come across mounjaro injections specifically and want to understand what the evidence says about how they work, the guide to m injection for weight loss addresses the medicine in detail so you can assess the claims yourself.

Side effects, safety monitoring and when to get medical help

The side-effect profile of both licensed peptide injections is led by the gut. Nausea, loose stools, constipation, reflux and burping are the most reported, typically most noticeable after starting treatment or after a dose step, and settling over days to a couple of weeks for most people. Fatigue, headache and injection-site reactions also appear in the trial data. The NHS tirzepatide patient page lists common and uncommon side effects clearly and is worth reading before you begin.

Pancreatitis is a less common but serious risk. The MHRA issued a Drug Safety Update in January 2026 flagging acute pancreatitis as a known, infrequent but potentially severe effect of GLP-1 medicines. Severe stomach pain (particularly if it spreads to the back or is accompanied by vomiting and does not ease) warrants urgent medical attention, not waiting for your next check-in. Similarly, signs of a serious allergic reaction, significant dehydration from vomiting or diarrhoea, or symptoms of gallbladder disease need prompt assessment by a clinician.

Both medicines carry the Black Triangle symbol (▼), meaning the MHRA collects additional safety data on them. Suspected side effects can be reported directly through the Yellow Card scheme. At nume, every repeat prescription goes through clinical re-review by a GPhC-registered prescriber before it is issued, if something has changed since your last order, it is caught at that stage. For a broader picture of how these injections sit within the wider treatment landscape, the weight loss treatments page sets out the full range of options available through our pharmacy.