Body weight loss injections: what to weigh up before you start

Which injection, and how do they differ?

The two body weight loss injections licensed in the UK are semaglutide, sold as Wegovy, and tirzepatide, sold as Mounjaro. Semaglutide activates the GLP-1 receptor, a pathway involved in appetite signalling and blood-sugar regulation. Tirzepatide activates two receptors, GLP-1 and GIP, which is why it is described as a dual agonist. That dual mechanism is believed to account for the larger average weight reductions seen in trials.

In the SURMOUNT-1 trial, tirzepatide at the 15mg dose produced average weight losses of around 20–21% over 72 weeks, as reported in the New England Journal of Medicine. Semaglutide at 2.4mg produced around 15% over 68 weeks in its own phase-3 programme. A direct head-to-head trial, SURMOUNT-5, confirmed that tirzepatide outperformed semaglutide on average weight loss, though both produced clinically meaningful results. Which figure applies to you, if either, is something no content page can answer. That is the prescriber's job.

There is also a practical question of preference. Both are once-weekly injections into the abdomen, thigh, or upper arm, you rotate sites. Some people find the injection routine straightforward within a few weeks; others take longer to feel comfortable. Our prescribers regularly hear that the idea of injecting felt daunting at first and became unremarkable by week three, and if you want to read more about how the weight loss injection r option fits into that picture, that page covers the detail. Worth knowing before the needle puts you off entirely.

Who these medicines are licensed for

Both Mounjaro and Wegovy are licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, raised cholesterol, type 2 diabetes, prediabetes, or obstructive sleep apnoea. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance. BMI alone does not determine clinical suitability, a prescriber also considers your medical history, current medicines, and any contraindications before approving treatment.

These medicines are not licensed for use in pregnancy, breastfeeding, or if you are actively trying to conceive. They are not licensed for anyone under 18. Conditions including a personal or family history of medullary thyroid carcinoma, or a history of pancreatitis, require careful discussion with a clinician before any treatment is considered. If you take oral contraceptives, the prescriber will advise you on additional contraceptive precautions, particularly with tirzepatide, because it can affect how quickly the pill is absorbed in the early weeks of treatment.

For a fuller picture of how the licensed injections compare across eligibility criteria, the NHS page on tirzepatide is a reliable starting point.

Side effects: what to expect and when to act

The most common side effects of both injections are gastrointestinal. Nausea is the one people mention most, followed by loose stools, constipation, indigestion, and reflux. These tend to be most pronounced in the first week or two after starting, or after a dose increase, and usually settle as your body adjusts. Fatigue and headache are also reported, particularly early on.

Most people manage by eating smaller portions, avoiding rich or fatty food early in treatment, and staying well hydrated. A minority find side effects persistent enough to pause or step back a dose, that is a clinical decision made with the prescriber, not a reason to stop treatment without advice.

There is a smaller category of side effects that need prompt medical attention. Severe, persistent stomach pain that spreads to the back (with or without vomiting) can be a sign of pancreatitis. The MHRA issued a Drug Safety Update in January 2026 reinforcing that this is a known but uncommon risk with GLP-1 medicines that warrants urgent care. Gallbladder symptoms, signs of a serious allergic reaction, and symptoms of severe dehydration from prolonged vomiting or diarrhoea also warrant same-day medical contact. If something feels wrong, speak to a doctor, not to our aftercare team first.

The access question: NHS, private, and what to watch out for

NHS access to these injections is available but restricted. NICE has recommended tirzepatide (TA1026) and semaglutide (TA875) for specific patient groups, and the NHS is rolling out tirzepatide through GP practices from 2025 onward, with eligibility broadening in phases. Many people either do not meet current NHS criteria or face significant waiting times, and our page on the m injection for weight loss explains how private access to that option works in practice. Private treatment through a regulated pharmacy is the practical alternative, and one worth approaching carefully.

These are prescription-only medicines. Any seller offering them without a clinical assessment, or without linking to a verifiable GPhC-registered pharmacy, is operating outside the law. The MHRA has seized large quantities of counterfeit and illegally sold weight-loss pens in recent years, including fake tirzepatide pens containing unknown substances. You can check any online pharmacy on the GPhC register in under a minute. For context on what private treatment typically costs and what a legitimate price includes, our pricing overview covers the detail honestly.

At nume, our pharmacy is GPhC-registered (premises number 9012878) and every consultation is personally reviewed by a GPhC-registered Independent Prescriber on the same day, a real clinician reads your answers, not software. For more on how these medicines work inside the body, or to explore the full range of treatment options we offer, those pages go into greater depth. When you are ready to find out whether you are clinically suitable, the right next step is a free consultation with our team.

You can start your free consultation here, our prescribers will take it from there.