Weight loss injections: what are they and how do they work?

What do weight loss injections actually do inside the body?

Your gut produces hormones after a meal that signal fullness to the brain, slow stomach emptying and help regulate blood sugar. If you are wondering what the weight loss injections are and how they replicate this process pharmacologically, the distinction between options matters. Semaglutide (Wegovy) targets one of those hormone pathways (GLP-1) while tirzepatide (Mounjaro) activates two simultaneously: GLP-1 and GIP. That dual action is why tirzepatide is sometimes described as more potent; it engages a broader part of the appetite-regulation system. To understand more about the specific effects on the body and appetite, the detail on what weight loss injections do is worth reading alongside this page.

The practical result for most people is that hunger arrives later, portions feel satisfying at a smaller size, and the pull towards high-calorie foods tends to ease. None of that is willpower, it is the hormonal environment shifting. Weight management researchers have understood for some time that obesity is a chronic biological condition, not a failure of character, and these medicines are the clearest pharmacological proof of that.

Both medicines are injected under the skin (typically into the abdomen, thigh or upper arm) once a week, on the same day each week where possible. The injection itself is small-gauge; most people describe it as barely noticeable. Whether weight loss injections are safe for you specifically is always a clinical question, but both carry extensive trial data supporting their safety profile when used under medical supervision.

Are there different types, and how do they compare?

Yes. The two licensed options work differently enough that they are not interchangeable without clinical input. Wegovy (semaglutide) has been available in the UK longer, with a well-established titration path from 0.25 mg up to a 2.4 mg maintenance dose, and more recently a 7.2 mg formulation approved by the MHRA in April 2026 for adults with a BMI of 30 or above. Mounjaro (tirzepatide) received its UK weight-management licence later and reaches up to 15 mg, with NICE recommending it in December 2024 under Technology Appraisal TA1026.

In the SURMOUNT-5 head-to-head trial, published in the New England Journal of Medicine in 2025, tirzepatide produced greater average weight reduction than semaglutide 2.4 mg over 72 weeks in adults with obesity and no diabetes. That said, the right medicine for a given person depends on their health history, any other medicines they take and how they respond to treatment, not simply on which trial figure is larger. A page comparing the two weight loss injections goes into more detail on the differences.

For those curious about oral options, Wegovy tablets received MHRA approval in June 2026 as the first oral GLP-1 medicine licensed for weight management in the UK, a separate topic, but worth knowing these medicines are no longer injection-only if that matters to you. The full picture of which weight loss injections are available in the UK covers both injectables and the newer tablet format.

Who can get them, and what does the assessment involve?

Both medicines are licensed for adults. Tirzepatide is licensed for those with a BMI of 30 or above, or 27 or above with at least one weight-related health condition such as high blood pressure, high cholesterol, obstructive sleep apnoea or prediabetes. Semaglutide carries a similar threshold. Lower BMI cut-offs apply for people from certain ethnic backgrounds under UK clinical guidance, your prescriber will confirm the threshold relevant to you.

BMI alone does not decide anything. A prescriber reviews your full picture: current medicines, relevant medical history, any contraindications (including pregnancy, breastfeeding or a history of certain conditions such as medullary thyroid cancer), and whether the expected benefit justifies starting treatment. That assessment is the legal and clinical gate through which every legitimate prescription passes. There are no shortcuts, and any seller offering these medicines without one should be avoided.

One check you can do in under a minute: before using any online pharmacy, look it up on the GPhC register of pharmacies. If the pharmacy is not listed, stop. Counterfeit weight loss pens have caused serious harm in the UK (the MHRA has seized large quantities of fake pens sold online) and registration is the fastest way to confirm you are dealing with a legitimate, regulated dispensary.

At nume, every consultation is read and assessed by a GPhC-registered Independent Prescriber on the same day. No algorithm handles the clinical review. If you are approved, treatment is dispatched the same day for free next-working-day delivery. Pricing, what is included and how the process works are set out on the treatment page.

What about side effects and what to watch for?

The most commonly reported side effects are gastrointestinal: nausea, loose stools, constipation, indigestion and burping. They tend to be most noticeable in the first days after starting or after a dose increase, and for most people they settle as the body adjusts. Starting at a low tolerability dose (the first pen exists to help your system adapt, not to produce weight loss immediately) is specifically designed to reduce their intensity.

More serious but less common effects include pancreatitis. In January 2026 the MHRA issued a Drug Safety Update flagging acute pancreatitis as a known risk with GLP-1 medicines: if you develop severe, persistent stomach pain that spreads to your back, with or without vomiting, seek urgent medical help rather than waiting for your next check-in. Gallbladder problems and dehydration from severe GI illness also warrant medical attention. The NHS tirzepatide medicines page lists the full side-effect profile clearly.

These are prescription medicines precisely because the balance of benefits and risks needs professional oversight. Side effects you are unsure about (whether they are expected, temporary or worth reporting) are exactly the kind of question to bring to your prescriber or aftercare team, not to manage alone. You can also report suspected side effects directly at yellowcard.mhra.gov.uk. Ongoing clinical support is part of what a well-structured weight loss programme should include, not an optional extra.