Weight Loss Injectables: What the Clinical Evidence Shows

What the large trials found about weight loss injectables

The clearest starting point is the published evidence. In SURMOUNT-1, a 72-week randomised trial involving 2,539 adults with obesity and no diabetes, participants taking tirzepatide at the highest dose lost an average of around 20–21% of their body weight, a result published in the New England Journal of Medicine that prompted NICE to recommend tirzepatide for NHS use in December 2024. For context, that is roughly three to four times the average sustained loss seen with lifestyle intervention alone in comparable populations.

Semaglutide 2.4mg (Wegovy) produced around 15% average weight reduction over 68 weeks in the STEP 1 trial, and a more recent 72-week trial of the 7.2mg dose reported approximately 20.7% average loss, narrowing the gap with tirzepatide considerably. In the SURMOUNT-5 head-to-head trial, published in 2025, tirzepatide produced greater average weight loss than semaglutide 2.4mg over 72 weeks in adults with obesity, the first direct comparison of the two medicines at those doses.

These are averages across trial populations; individual results vary, and no outcome is guaranteed. What the data consistently shows is that both medicines produce clinically significant weight reduction in the people who complete treatment alongside the recommended lifestyle support. If you want a fuller breakdown of how the two options compare, our comparison of which injectable suits which patient goes through the evidence side by side.

How injectable weight loss medicines actually work in the body

Both medicines work by mimicking hormones released naturally after eating. Semaglutide activates GLP-1 receptors in the brain and gut, signalling fullness, slowing the rate at which the stomach empties and reducing appetite. Tirzepatide adds a second pathway: it activates GIP receptors as well, making it the only dual-agonist injectable weight loss drug currently licensed in the UK. The practical effect of both is that people eat less without feeling deprived in the way that calorie restriction alone typically produces.

They do not burn fat directly. Weight loss happens because energy intake falls substantially, most people find their appetite and portion sizes reduce significantly over the first few weeks of treatment. That reduction in drive to eat is why the trial results look so different from those of older diet pills.

The injections are administered once a week, subcutaneously (just under the skin), rotating between the abdomen, thigh and upper arm. Neither requires a clinic visit: both come as pre-filled pens designed for self-injection at home. For practical guidance on the pen format itself, our page on the injectable pen covers the injection steps, storage and what to check before each dose. Proper site rotation and keeping the pen refrigerated between uses matter more than many people realise at the start.

Who the licensed criteria cover, and what the prescriber weighs up

Under the UK product licences, both medicines are indicated for adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. For people from South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds, those thresholds are typically 2.5 kg/m² lower under UK guidance. The NICE technology appraisal for tirzepatide (TA1026) sets its own eligibility criteria for NHS access, which are narrower than the licensed indication and currently limited to specific BMI and comorbidity thresholds introduced in phases from June 2025.

BMI alone does not determine suitability. A prescriber also considers your medication history, any history of pancreatitis or thyroid conditions, current contraception (particularly relevant for tirzepatide, which can affect oral contraceptive absorption in the first four weeks of treatment and after each dose increase), and other factors specific to you. These medicines are not recommended during pregnancy, while breastfeeding, or for anyone trying to conceive, and they are not licensed for under-18s.

For the broader picture of how treatment options sit alongside each other, our treatments page explains the clinical pathway. You can also read more about the range of injectable drugs available for weight loss and how the licensing differs between them, including why Ozempic (also semaglutide) is licensed for type 2 diabetes rather than weight management.

Side effects, safety signals and what to watch for

The most common side effects with both injectables are gastrointestinal: nausea, loose stools, constipation, indigestion and burping appear most often in the first weeks after starting or after moving to a higher dose. For most people they are mild to moderate and settle as the body adjusts. Eating smaller portions, avoiding very fatty meals and staying well hydrated all help.

More serious effects are less common but matter. In January 2026, the MHRA issued a Drug Safety Update highlighting acute pancreatitis as an infrequent but potentially serious risk with GLP-1 medicines: severe, persistent stomach pain that spreads to the back (with or without vomiting) needs prompt medical attention rather than waiting it out. Gallbladder problems and symptoms of dehydration from prolonged vomiting or diarrhoea also warrant a call to a clinician rather than self-management. Suspected side effects can be reported through the MHRA's Yellow Card scheme.

On the practical side, each used pen contains a needle and should be disposed of in a sharps bin rather than household waste, a 1-litre sharps container is a straightforward solution for home use. If you have questions about what to do if a dose is missed or when to contact your prescriber, our FAQs cover the questions our clinical team hears most often. Cost context (including how private treatment is typically priced and what an all-in price should cover) is explained on our page on the cost of weight loss injections.

If you are thinking about starting treatment, our clinical team at nume reviews every consultation personally, a named, GPhC-registered prescriber reads your answers the same day, not software. Speak to our prescribers to find out whether weight loss injectables are suitable for you.