Clear information on weight loss injections in the UK

Step 1: understanding what each injection actually does in your body

Wegovy contains semaglutide, made by Novo Nordisk. It is a GLP-1 receptor agonist, a medicine that mimics a gut hormone released after eating, slowing gastric emptying, reducing appetite and helping to regulate blood sugar. You can find the patient-level detail on the NHS semaglutide medicines page.

Mounjaro contains tirzepatide, made by Eli Lilly. It works on both the GIP and GLP-1 receptors (two distinct gut-hormone pathways) which is why it is sometimes described as a dual-agonist. In clinical trials, this dual action produced greater average weight loss than single-agonist medicines in head-to-head comparisons, though individual responses vary. The Mounjaro weight loss injection page covers the mechanism in more depth.

Neither medicine burns fat directly. What they do is make it substantially easier to eat less, by changing the hormonal signals that govern hunger and fullness. Alongside a reduced-calorie diet and regular activity, that shift in appetite is where the clinical effect comes from. Treatment starts at the lowest strength (2.5 mg for Mounjaro, 0.25 mg for Wegovy) to let your system adjust before the prescriber considers increasing the dose in approximately four-week steps.

Step 2: checking whether you are likely to be eligible

Both injections are licensed for adults with a BMI of 30 or above, or a BMI of 27 or above if you have at least one weight-related health condition, for example, high blood pressure, type 2 diabetes, high cholesterol, obstructive sleep apnoea or osteoarthritis. For some ethnic backgrounds, the relevant thresholds are set 2.5 kg/m² lower under UK guidance; the NHS BMI calculator is a useful starting point.

A BMI figure on its own does not determine suitability. The prescriber looks at the whole picture: your medical history, current medicines, any contraindications and your broader health goals. Neither injection is licensed for use in pregnancy, during breastfeeding, or if you are actively trying to conceive. They are not licensed for people under 18.

On the NHS, tirzepatide and semaglutide are available through a phased rollout with strict eligibility thresholds and, in many cases, significant waiting times. A private prescription route exists for people who do not meet the NHS criteria or prefer not to wait, provided clinical assessment confirms suitability. That assessment is the non-negotiable starting point either way.

Step 3: what using the injection actually looks like week to week

Both medicines arrive as pre-filled pens. Mounjaro uses a device called the KwikPen; each pen contains four weekly doses. Wegovy comes in individual pre-filled pens, one per week. You inject subcutaneously (just under the skin) into the abdomen, outer thigh or upper arm, rotating the site each week to avoid irritating the same spot repeatedly.

The technique is straightforward, and most people feel confident with it quickly. Practical questions about injection preparation come up often; for site-cleaning between injections, alcohol swabs are the standard recommendation in clinical guidance, and used needles and pens should go into an approved sharps container for safe disposal, your local GP surgery or pharmacist can advise on collection points.

Storage matters: both pens are refrigerated at 2–8°C before first use. The exact room-temperature window for in-use pens differs between the two medicines and between pen formats; always follow the Patient Information Leaflet that comes with your specific prescription rather than figures given online.

Step 4: side effects to be aware of, and when to seek help quickly

The most commonly reported side effects are gastrointestinal: nausea, loose stools, constipation, indigestion, burping and, less often, vomiting. These tend to cluster around the start of treatment or after a dose increase, and often ease within a couple of weeks as the body adjusts. Fatigue, headache and injection-site reactions are also reported.

Most side effects are mild to moderate and manageable with practical changes, eating smaller portions, eating slowly, avoiding very fatty meals and drinking plenty of water. The three licensed weight loss injections page compares side-effect profiles in more detail.

A small number of people experience more serious reactions. Severe, persistent stomach pain that radiates towards the back (with or without vomiting) should be assessed by a doctor promptly, as it can be a sign of pancreatitis. The MHRA highlighted acute pancreatitis as a known but infrequent risk in a formal Drug Safety Update in January 2026. Gallbladder symptoms, signs of dehydration from prolonged vomiting or diarrhoea, and allergic reactions also warrant prompt medical attention. Any suspected side effect can be reported to the MHRA through the Yellow Card scheme.

For a broader look at what weight loss medication by injection involves from a cost and access perspective, including what a private prescription typically covers, there is dedicated guidance on the service page. If you are specifically considering Wegovy, our Wegovy weight loss injection page brings together the clinical detail, dosing schedule and eligibility information in one place. When you are ready to find out whether one of these treatments is clinically suitable for you, check your eligibility with our prescribers, it is a free consultation, reviewed the same day by a GPhC-registered Independent Prescriber, and our weight loss injections overview can help you compare your options beforehand.