The truth about weight loss injections: what the evidence actually shows

The results are significant, and they have limits

In SURMOUNT-1, adults taking tirzepatide at the highest dose lost around 20–21% of their body weight on average over 72 weeks. STEP 1 showed approximately 15% average weight loss over 68 weeks with semaglutide 2.4mg. Those are average figures across thousands of participants: some people lost considerably more, some less. Biology, starting weight, how closely people stuck to the recommended diet and activity changes, and how long they stayed on treatment all played a part.

A 2025 head-to-head trial called SURMOUNT-5 compared the two directly in adults with obesity and no diabetes over 72 weeks. Tirzepatide produced greater average weight reduction. The gap narrows at the highest semaglutide dose (7.2mg, approved by the MHRA in early 2026), but the direction of the comparison held. If you want to understand all the weight loss injections currently available in the UK, that context is useful before deciding which clinical conversation to have. Knowing this is useful context; it doesn't tell you which medicine suits your situation, that's a clinical conversation, not a league table to scroll through.

One practical thing worth doing before reading further: check whether any pharmacy or clinic offering these medicines appears on the GPhC pharmacy register. It takes under a minute and is the single most reliable signal that a service is operating legally.

The mechanism isn't a mystery, but the 'skinny jab' framing misses most of it

'Skinny jab' became a tabloid shorthand that flattened something genuinely interesting. These medicines aren't stimulants, diuretics or fat-blockers. Tirzepatide, the active ingredient in Mounjaro, mimics two hormones (GIP and GLP-1) that the gut releases in response to food. The result is that appetite signals change, the stomach empties more slowly, and the sense of fullness arrives earlier and lasts longer. Semaglutide (Wegovy) does something similar through the GLP-1 receptor alone.

That biological shift is why NHS information on tirzepatide frames it as a medicine for a health condition (obesity) rather than a cosmetic shortcut. The MHRA has said clearly that GLP-1 medicines are not assessed or intended for quick or cosmetic weight loss. Both Mounjaro and Wegovy are licensed alongside a reduced-calorie diet and increased physical activity; the injection doesn't replace those changes, it makes them more achievable by reducing the biological drive to eat more than you need. Framing weight as a willpower problem misses the hormonal and metabolic picture that these medicines are designed to address.

If you're trying to understand which medicine might be appropriate for you, it helps to first get a clear answer to what the weight loss injections actually are before comparing them side by side with a prescriber.

Side effects, safety and what to watch for

The most common side effects (nausea, vomiting, diarrhoea, constipation, indigestion, reflux) follow a pattern. They tend to be most noticeable at the start of treatment or after a dose increase, and for most people they ease within days to a couple of weeks as the body adjusts. Injection-site reactions, fatigue, headache and dizziness appear less commonly.

More serious but less frequent risks include acute pancreatitis. In January 2026 the MHRA issued a Drug Safety Update on GLP-1 medicines specifically highlighting this: severe, persistent stomach pain that spreads to the back, with or without vomiting, needs urgent medical assessment. Gallbladder problems and signs of dehydration from prolonged vomiting or diarrhoea also warrant prompt attention. None of this is a reason to avoid treatment if you're clinically suitable, it's a reason to be supervised by someone who knows your history and checks in regularly. Suspected side effects can be reported directly to the MHRA at the Yellow Card scheme.

Women using oral contraceptives should be aware that tirzepatide may reduce how well the pill is absorbed during the first four weeks of treatment and for four weeks after each dose increase; adding a barrier method during those windows is the standard advice. The same concern hasn't been shown with semaglutide, though it's worth discussing with your prescriber if you're switching between the two.

How legitimate supply works, and why counterfeits matter

Both medicines are prescription-only under UK law. Getting them legally means a prescriber has assessed you, determined you meet the licensed criteria, and issued a prescription through a regulated pharmacy. If you are researching weight loss injections and want to understand what a legitimate clinical pathway looks like, that process is worth understanding before you approach any service. Mounjaro and Wegovy are not available over the counter, and they cannot be legally sold without that prescriber involvement.

The counterfeit market has grown sharply alongside demand. In October 2025 the MHRA raided a site in Northampton manufacturing fake tirzepatide pens, having already seized around 20 million doses of illegally traded weight-loss medicines with an estimated street value of around £45 million. Earlier seizures of fake Ozempic pens found some contained insulin rather than semaglutide, a potentially dangerous substitution. A low price, a social-media listing, or a seller offering the medicine without a prior clinical assessment are all red flags. The MHRA's Yellow Card scheme accepts reports of suspect sellers as well as side effects.

For a fuller picture of how pricing works in the private market, the guide to weight loss injection costs in the UK explains what legitimate pricing typically includes and what to question. People who are also searching for weight loss injections in my area will find that checking a provider against the GPhC register is the most reliable first step before booking anything local. If you'd like to understand the broader range of treatment options beyond injections, our weight loss treatment overview is a good starting point.

At nume, every consultation is read by a GPhC-registered Independent Prescriber on the day it's submitted. Medicines are sourced through the licensed UK supply chain. If you'd like to find out whether treatment is suitable for you, start your free consultation and a prescriber will review your answers the same day.