The new weight loss injections of 2024: what the evidence showed

What the major trials actually reported — and why the numbers matter

The clearest picture of how these medicines perform comes from two large published trials. The STEP 1 trial, published in the New England Journal of Medicine, followed adults with obesity or overweight plus a weight-related condition for 68 weeks on semaglutide 2.4mg. Average weight loss was around 15% of body weight. That was a significant result. Then SURMOUNT-1 reported on tirzepatide across the full dose range: at 15mg, average weight loss reached approximately 20–21% over 72 weeks, with some analyses reaching 22.5%, figures that had not been seen for a licensed weight-loss medicine in the UK before. SURMOUNT-1 enrolled 2,539 adults, and the wider tirzepatide clinical programme across both weight and diabetes studies involved more than 10,000 participants in total. These are not small signals.

The practical implication is that both medicines produce meaningful, sustained weight reduction when combined with dietary changes and increased activity, but they are not equivalent. SURMOUNT-5, a head-to-head trial published in 2025, confirmed that tirzepatide produced greater average weight loss than semaglutide 2.4mg over 72 weeks. Which medicine suits a given person, though, is a clinical judgement, not a league-table exercise.

How tirzepatide and semaglutide worked differently, and what that meant in practice

Semaglutide is a GLP-1 receptor agonist: it mimics a gut hormone that signals fullness, slows gastric emptying and reduces appetite. Tirzepatide does all of that and also activates GIP receptors, a second gut-hormone pathway involved in appetite regulation and blood-sugar control. That dual mechanism is why tirzepatide was described as the first of its kind when NICE appraised it in December 2024.

In everyday terms, both medicines meant most people felt full faster and stayed full longer. Appetite often changed noticeably within the first few weeks. Side effects were mostly gastrointestinal (nausea, loose stools, constipation, indigestion) and tended to be most noticeable after the first pen or after a dose increase, settling for most people within days to a couple of weeks. You can read a thorough rundown on the weight loss injections overview page.

One practical detail patients often raise: tirzepatide is stored in the fridge, ideally somewhere accessible like the door shelf, and the Patient Information Leaflet sets out a limited window for room-temperature storage if you need to travel. Refer to the leaflet or your prescriber rather than an online summary for the exact figures.

Who was eligible in 2024, and what the licensed criteria said

Both medicines were licensed for 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. NICE's appraisal of tirzepatide (TA1026), published in December 2024, set a recommended threshold of BMI 35 or above with at least one weight-related comorbidity for NHS use, with lower thresholds applying for some ethnic backgrounds. Private prescribing follows the licensed criteria rather than NICE's commissioning thresholds, so the eligibility picture looked different depending on route.

Neither medicine is licensed for under-18s. Use during pregnancy, breastfeeding or when trying to conceive is not recommended. A history of certain conditions, including medullary thyroid carcinoma or pancreatitis, requires careful prescriber discussion before starting. BMI is a starting point, not the whole picture, a prescriber weighs your full medical history. If you want to understand whether you might be eligible, the eligibility guide for the new weight loss injection covers this in more detail.

Private routes, NHS access, and what legitimacy actually looks like

Through 2024, NHS access to these medicines was limited. Wegovy was available only via specialist weight management services with long waiting lists. Tirzepatide received its NICE recommendation in December 2024, with phased NHS rollout beginning from mid-2025. The result was that most people who started treatment in 2024 did so through a private online pharmacy.

That created a real problem alongside a real opportunity. The MHRA warned repeatedly about counterfeit pens circulating online, sold without clinical oversight. Fake pens have in some cases contained insulin, with serious consequences for people without diabetes. The safe route is through a pharmacy you can verify on the GPhC register: a regulated premises, a GPhC-registered prescriber reviewing your consultation, and medicine sourced through the licensed UK supply chain. Red flags include sellers offering these medicines without a clinical assessment, prices dramatically below market rate, or discount codes attached to prescription medicines, none of which belong in a legitimate supply chain.

For context on cost across providers, the treatment options page sets out what a legitimate private prescription typically includes. At nume, consultations are reviewed the same day by a real prescriber, not software, and if treatment is clinically suitable, the pen is dispatched the same day for free next-working-day tracked delivery. Our about us page explains how the service is structured, and you can verify our GPhC-registered pharmacy directly at the register (premises number 9012878). The newest developments since 2024, including the Wegovy 7.2mg pen and the first oral GLP-1 approved in the UK, are covered on the 2025 update page.

If you're weighing up your options, this guide to the newer injectable medicines and our frequently asked questions are a useful next step before speaking to a prescriber, and if you're based in Idaho you may also find our guide to weight loss injections in Boise helpful alongside a look at what's covered on the new weight loss injection page.