Which injection helps with weight loss, and how does it work?

What does the injection actually do inside your body?

A question our prescribers hear most weeks is whether these injections "burn fat directly". They do not. What they do is change the hormonal signals that drive hunger and dictate how quickly your stomach empties after a meal. Semaglutide mimics a gut hormone called GLP-1, which is released naturally when you eat. It tells the brain you are full, slows food moving through the stomach, and reduces the urge to eat between meals. Tirzepatide does the same, and then adds a second pathway: it also activates GIP receptors, which are involved in both appetite regulation and how the body handles blood sugar. That dual action is why tirzepatide is sometimes described as the only dual-agonist weight-loss medicine licensed in the UK.

The practical effect for most people is a meaningful reduction in appetite that makes eating less feel less effortful. Many people also find their interest in high-calorie foods fades. You can read more about the broader range of weight-loss injections and how they sit within the landscape of medical weight management.

How much weight do these injections help people lose in trials?

Trial data is the clearest measure we have, and both medicines performed well. In SURMOUNT-1, participants taking the highest tirzepatide dose lost around 20–21% of their body weight on average over 72 weeks, alongside lifestyle support. The STEP 1 trial for semaglutide 2.4mg reported around 15% average loss over 68 weeks. A direct head-to-head comparison (the SURMOUNT-5 study, published in the New England Journal of Medicine in 2025) found tirzepatide produced greater average weight reduction than semaglutide 2.4mg in adults with obesity who did not have type 2 diabetes, and if you are interested in how these medicines were originally developed you may find our page on the diabetic injection that helps with weight loss a useful read.

These are averages across large groups. Individual results vary, and neither medicine guarantees a specific outcome. The NICE appraisal of tirzepatide notes that people who have not lost at least 5% of their starting weight after six months at the highest tolerated dose should have their treatment reviewed, you can read the full NICE guidance on tirzepatide (TA1026) for the clinical criteria in detail. For context on what treatment typically costs privately, the weight-loss injection prices page covers what influences the figures you will see quoted.

Who can be prescribed a weight-loss injection?

Both medicines carry similar starting thresholds under their UK licences. Adults with a BMI of 30 or above are eligible to be considered; those with a BMI between 27 and 29.9 may also qualify if they have at least one weight-related health condition, high blood pressure, raised cholesterol, prediabetes, type 2 diabetes, and obstructive sleep apnoea are among the conditions that count. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance. A BMI figure on its own does not secure a prescription: a prescriber looks at your full health history, current medicines, and any contraindications before deciding whether and which treatment is appropriate. You can check broadly what weight criteria apply before starting a consultation.

There are groups for whom these medicines are not recommended: people who are pregnant, breastfeeding, or actively trying to conceive should not use them. Neither medicine is licensed for under-18s. Anyone with a personal or family history of certain thyroid conditions should discuss this with their prescriber before treatment begins. If you are curious whether conditions like PCOS might affect your suitability, our guide on injections and PCOS goes into more detail.

What are the most common side effects to expect?

Most side effects sit in the digestive system: nausea, loose stools, constipation, indigestion, and burping are the most frequently reported. For many people they are most noticeable in the first week or two after starting, or after a dose increase, and then settle. Fatigue and mild headache are also common early on. Injection-site reactions (redness or a small lump where you injected) can happen and usually resolve quickly. The pen itself rotates between sites on the abdomen, thigh, or upper arm.

One thing worth knowing before you start: nausea is common but rarely severe enough to stop treatment. The starting dose on both medicines exists partly to give your system time to adjust before any increase. Serious side effects are uncommon but real, severe stomach pain that spreads to the back should be treated as urgent and you should seek medical help promptly, as this can occasionally signal pancreatitis. Report any suspected side effect, including anything unexpected, via the MHRA Yellow Card scheme. For a full picture of what to expect, the weight-loss injection overview covers side effects alongside practical injection guidance.

Thinking about whether this could be right for you? Speak to our prescribers, a free consultation with a GPhC-registered clinician is the clearest next step, and your consultation is reviewed the same day it is submitted at weight-loss treatments.