Injections to help weight loss: what the clinical evidence actually shows

What the large trials established, and why the numbers matter

The STEP 1 trial, published in the New England Journal of Medicine, randomised adults with obesity to semaglutide 2.4mg or placebo over 68 weeks alongside lifestyle changes. Average weight reduction in the semaglutide group was around 15%. That figure reshaped clinical expectations of what a non-surgical medicine could achieve.

Tirzepatide then raised the bar. SURMOUNT-1, involving 2,539 adults with obesity, reported average body-weight reductions of around 20–21% at the 15mg dose over 72 weeks. In a direct head-to-head trial (SURMOUNT-5, published in the New England Journal of Medicine in 2025), tirzepatide produced greater average weight loss than semaglutide 2.4mg. These are not marketing figures, they come from the same peer-reviewed process that underpins NICE's decision to recommend both medicines.

What the trials also showed is that results depend heavily on dosing and persistence. Most participants started well below the maintenance dose, titrating gradually to reduce gastrointestinal side effects. The weight loss accumulated over months, not days. Understanding the trajectory matters: if you're wondering exactly how weight-loss injections drive fat loss, the mechanism is better explained than most people expect, appetite suppression, slower gastric emptying, and changes in energy regulation all play a role.

Which adults these injections are licensed and recommended for

Licensing thresholds under the UK SmPCs are: adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related health condition such as high blood pressure, type 2 diabetes, high cholesterol, or obstructive sleep apnoea. A lower BMI threshold (typically 2.5 kg/m² less) applies for people from some ethnic backgrounds under UK guidance.

NICE's recommendations sit within those licensed parameters but with additional criteria. NICE TA1026 recommends tirzepatide for adults with a BMI of 35 or above and at least one weight-related comorbidity. NICE TA875 recommends semaglutide within specialist weight management services, for a maximum of two years, at similar thresholds. These NHS-route criteria are stricter than private licensing, a point that explains why many people turn to regulated private pharmacies rather than waiting for NHS access.

BMI alone never settles the question. A prescriber reviews the whole picture: current medications, medical history, contraindications, and whether the clinical balance favours treatment. People with a personal or family history of medullary thyroid carcinoma, certain pancreatic conditions, or who are pregnant, breastfeeding, or trying to conceive are not suitable candidates. The full eligibility picture for weight-loss injections covers these exclusions in more detail.

The practical side of using these injections (what patients wish they'd asked first

A question our prescribers hear most weeks runs something like: "I've started the injection but I'm not losing much weight yet) is that normal?" It almost always is. Treatment begins at a starter dose that exists to let your body adjust, not to produce immediate weight loss. Dose increases happen in roughly four-week steps, and meaningful weight reduction typically builds through the middle and later months of treatment.

Both Mounjaro and Wegovy are injected once weekly into the abdomen, thigh, or upper arm, with the injection site rotated. Storage is refrigerated. If you want to understand the routine of keeping and using the pen correctly, the general weight-loss injections overview covers handling and storage basics, and the patient information leaflet for your specific medicine contains the definitive instructions.

Side effects tend to be gastrointestinal: nausea, loose stools, constipation, reflux, and occasional fatigue or headache are the most commonly reported. Most people find them manageable, particularly if they eat smaller portions and avoid very fatty foods during the early weeks. They usually ease as the body adjusts. Severe or persistent abdominal pain that radiates to the back warrants urgent medical attention, the MHRA has flagged acute pancreatitis as an infrequent but serious risk with GLP-1 medicines, and the Yellow Card scheme exists for reporting any suspected side effects.

For people asking about related treatments (for example, whether B12 injections help with weight loss, or whether there are options for specific conditions like PCOS) the evidence picture is quite different from the licensed GLP-1 medicines. The evidence on injections and PCOS is worth reading separately if that's relevant to you.

Accessing injections safely: the prescription route and what to verify

Mounjaro and Wegovy are prescription-only medicines. There is no legal route to them in the UK without a clinical assessment and a prescription issued by a qualified prescriber. That's not bureaucracy for its own sake, it's the structure that keeps these medicines safe, because contraindications, drug interactions, and appropriate dosing all require clinical judgement.

Online supply has grown rapidly, and with it the circulation of counterfeit pens. The MHRA has seized large quantities of fake weight-loss medicines sold without clinical oversight. Verifying any online pharmacy on the GPhC pharmacy register takes about thirty seconds and is the single most reliable check you can do. If a supplier offers these medicines without asking you to complete a clinical consultation, that's a clear red flag. A brief overview of what to look for is on our guide to getting weight-loss injections from a registered pharmacy.

Cost is also a factor worth understanding clearly. Private prices vary by dose and provider, and the Eli Lilly list price increase in September 2025 pushed the range for the highest Mounjaro dose to around £330 per four-week pen at list price; typical private market prices since then have sat roughly in the £149–£375 per month range depending on dose and provider. The cost context for weight-loss injections explains what legitimate pricing typically includes, and why a price that looks unusually low deserves scrutiny.

At nume, every consultation is reviewed by a GPhC-registered Independent Prescriber, a real clinician reads your answers the same day. If treatment is clinically suitable and approved before midday Monday to Friday, it's dispatched the same day and delivered free the next working day in plain, unbranded packaging by DPD. One price covers the consultation, prescription, treatment, and aftercare. Check your eligibility and start your free consultation here.