Weight loss injections are prescription medicines that work on your body's natural gut hormones to reduce appetite, so you feel fuller sooner and eat less. In the UK, the two main licensed options are Mounjaro (tirzepatide) and the Wegovy injection (semaglutide), both taken as a once-weekly self-injection alongside diet and lifestyle changes.
They are not a quick fix or a substitute for healthy habits. They are tools that make it easier to eat less, and they are prescribed only after a clinical consultation confirms they are appropriate and safe for you.
How weight loss injections work
Both licensed injections belong to a group of medicines that mimic gut hormones released after you eat. These hormones tell your brain you are full, slow down how quickly your stomach empties, and reduce food cravings.
The result is that you naturally feel satisfied with smaller portions and think about food less often. This is why appetite reduction is one of the most commonly reported effects.
The difference between the two injections
Wegovy contains semaglutide, a GLP-1 receptor agonist — it acts on a single gut-hormone pathway. It was the first medicine specifically licensed by the MHRA for weight management.
Mounjaro contains tirzepatide, which activates two gut-hormone receptors at once — GLP-1 and GIP. This dual action is what sets it apart from single-hormone medicines.
How well do they work?
Clinical trials give a useful benchmark, though your own results depend on your body, your dose and the lifestyle changes you make.
- Mounjaro: In the SURMOUNT-1 trial, participants on the 15mg dose lost on average 22.5% of body weight over 72 weeks, alongside lifestyle support. Individual results vary; not everyone reaches this.
- Wegovy: In the STEP UP clinical trial, adults on the 7.2mg dose lost on average 20.7% of their body weight over 72 weeks, alongside lifestyle support. Individual results vary; not everyone reaches this.
These are different trials, not a direct head-to-head comparison, so the figures should not be read as one medicine simply beating the other.
Dosing and how you start
Both medicines start low and increase gradually. This step-up approach, called titration, gives your body time to adjust and helps reduce side effects.
| Medicine | Starting dose | Maximum licensed dose |
|---|---|---|
| Mounjaro (tirzepatide) | 2.5mg | 15mg |
| Wegovy (semaglutide) | 0.25mg | 7.2mg |
Mounjaro moves through 2.5mg, 5mg, 7.5mg, 10mg and 12.5mg before the 15mg maximum. Wegovy moves through 0.25mg, 0.5mg, 1mg, 1.7mg and 2.4mg (the standard dose) up to the 7.2mg maximum. A prescriber reviews each step, and you stay at whichever dose suits you and is well tolerated.
Prefer a tablet instead of an injection?
Injections are not the only option. The Wegovy Tablet contains the same active medicine as the Wegovy injection (semaglutide) but is taken as a once-daily tablet. It must be taken first thing on an empty stomach with a small sip of plain water, waiting at least 30 minutes before eating, drinking anything else or taking other medicines — taken with food it is far less effective.
There is also Foundayo (orforglipron), a once-daily GLP-1 tablet that, unlike the others, can be taken at any time of day with or without food or water. It is the first small-molecule (non-peptide) GLP-1 pill. You can compare all options on our treatments overview.
Who can have weight loss injections?
Eligibility is consistent across the licensed treatments. You may be suitable if you are:
- Aged 18 or over; and
- Have a BMI of 30 or above (obesity); or
- Have a BMI of 27–30 with a weight-related condition such as type 2 diabetes, high blood pressure or sleep apnoea.
These medicines are not suitable if you are pregnant, planning pregnancy or breastfeeding, or if you have type 1 diabetes, a personal or family history of medullary thyroid carcinoma or MEN2, a history of pancreatitis, severe gastrointestinal disease, an active eating disorder, severe liver or kidney impairment, or a known allergy to the active ingredient.
A treatment is prescribed only if it is clinically appropriate after a free online consultation — a prescription is never guaranteed.
Side effects to be aware of
Because these medicines share the same GLP-1 class, they have a similar side-effect profile. The most common effects (affecting more than 1 in 10 people) are:
- Nausea
- Diarrhoea
- Constipation
- Vomiting
- Decreased appetite
- Fatigue
These are often worst when starting or increasing a dose and tend to settle as your body adjusts. This is exactly why the dose is built up slowly.
Serious but rare risks include severe allergic reaction, acute pancreatitis, severe hypoglycaemia (low blood sugar), severe gastrointestinal disease and worsening diabetic retinopathy. Seek medical help if you experience severe or persistent symptoms.
You can report any suspected side effect through the MHRA Yellow Card scheme at https://yellowcard.mhra.gov.uk/.
What do weight loss injections cost?
The online consultation is free — you only pay if a treatment is prescribed and you choose to proceed. Prices depend on the medicine and dose.
| Medicine | Price range per month |
|---|---|
| Mounjaro | From £160 (2.5mg) up to £295 (15mg) |
| Wegovy injection | From £115 (0.25mg) up to £245 (7.2mg) |
See the full dose-by-dose pricing on the Mounjaro and Wegovy pages, or view the combined weight loss injections overview.
How to start safely
Weight loss injections are Prescription-Only Medicines, so they should only be supplied by a properly regulated pharmacy after a genuine clinical review. At The Weight Clinic, the process is:
- A free online consultation about your health and weight.
- A GPhC-registered independent prescriber reviews your answers to check treatment is safe and appropriate.
- A short video consultation where clinically needed.
- If suitable, the medicine is dispensed by our own UK pharmacy and sent by tracked delivery, with ongoing prescriber support.
Our medicines are genuine, MHRA-licensed products dispensed by our GPhC-registered pharmacy — never unlicensed or grey-market imports. When choosing any provider, check the pharmacy is on the General Pharmaceutical Council register at pharmacyregulation.org.
Ready to see whether treatment could be right for you? Start your free consultation, browse the blog for more guidance, or contact us with any questions.
This article is for information only and is not a substitute for personalised medical advice. Weight-loss medicines are prescription-only, are not suitable for everyone, and are prescribed only after a clinical consultation with a GPhC-registered prescriber. Individual results vary. Always speak to a UK-registered pharmacist or doctor before starting or changing any prescription medication.
