A weight loss injection is a prescription-only medicine — mainly Wegovy (semaglutide) and Mounjaro (tirzepatide) in the UK — that reduces appetite by mimicking gut hormones involved in fullness. They’re available on the NHS for people who meet strict clinical criteria, and privately for a wider group, but they aren’t a quick fix: they’re intended to be used alongside diet and exercise changes and under medical supervision.
How Do Weight Loss Injections Work?
Wegovy and Ozempic contain semaglutide, which mimics a hormone called GLP-1 (glucagon-like peptide-1). This hormone tells the brain you’re full, slows down digestion, and helps regulate blood sugar. Mounjaro contains tirzepatide, which acts on GLP-1 and a second hormone, GIP, which is one reason trials show it producing somewhat greater weight loss on average.
All of these are given as a once-weekly injection, usually into the stomach, thigh or upper arm, with the dose increased gradually over several weeks under a clinician’s guidance.
Wegovy vs Mounjaro vs Ozempic: What’s the Difference?
| Medication | Active ingredient | Licensed for | Typical trial weight loss |
|---|---|---|---|
| Wegovy | Semaglutide | Weight management | Around 14–15% of body weight |
| Mounjaro | Tirzepatide | Weight management & type 2 diabetes | Around 20% of body weight |
| Ozempic | Semaglutide | Type 2 diabetes (weight loss is an off-label side effect) | Not licensed for weight loss specifically |
| Saxenda | Liraglutide | Weight management (daily injection) | Generally lower than Wegovy or Mounjaro |
Figures are approximate averages from clinical trials and vary by individual.
Can You Get Weight Loss Injections on the NHS?
Yes, but access is limited and phased. Broadly:
- Mounjaro can be prescribed by GPs and specialist clinics in England, generally starting with people who have a BMI of 40+ (or 37.5+ from some minority ethnic backgrounds) and at least four weight-related conditions such as type 2 diabetes, high blood pressure or sleep apnoea. Eligibility criteria are expected to widen over time as the rollout continues.
- Wegovy is accessed through specialist NHS weight management services rather than a standard GP prescription, generally requiring a BMI of 35+ (or 32.5+ for some ethnic groups) plus at least one weight-related condition.
Not every GP practice offers these yet, and availability depends on your local Integrated Care Board, so waiting times vary a lot by area.
How Much Do Weight Loss Injections Cost Privately?
If you don’t meet NHS criteria, both Wegovy and Mounjaro are available through private prescribers and online pharmacies in the UK, typically starting from around £150–£250 a month depending on the dose and provider, usually rising as the dose is increased. Private treatment still requires a consultation to check the medication is appropriate for you.
Side Effects and Safety
The most commonly reported side effects are gastrointestinal: nausea, diarrhoea, constipation and reflux, especially when starting or increasing the dose. These tend to ease over time. Both medications carry MHRA-reviewed safety data, but they aren’t suitable for everyone — people with a history of certain thyroid cancers, pancreatitis, or who are pregnant or breastfeeding, are generally advised against them, which is why a proper clinical consultation matters.
What Happens When You Stop Taking Them?
Weight loss injections manage appetite while you’re taking them; they don’t create a permanent change on their own. Research on semaglutide found that within a year of stopping, users regained a substantial majority of the weight they’d lost, unless diet and exercise habits built during treatment were maintained. This is a key reason clinicians pair injections with lifestyle support — such as a proper weight loss tracker and regular exercise like swimming — rather than prescribing them in isolation.
FAQs About Weight Loss Injections
What is the best weight loss injection?
Mounjaro (tirzepatide) has shown greater average weight loss than Wegovy in head-to-head trial data, but the right choice depends on individual health factors a prescriber will assess.
Can I get weight loss injections on the NHS?
Only if you meet specific BMI and health-condition criteria, and access is currently limited by area and phased rollout. Many people who want them don’t yet qualify.
How much weight can you lose with injections?
Clinical trials show average losses of around 14–15% of body weight with Wegovy and around 20% with Mounjaro over roughly 68–72 weeks, though individual results vary.
Do you regain weight after stopping weight loss injections?
Many people regain a significant portion of the weight within a year of stopping unless they maintain the diet and exercise changes made during treatment.