Best Weight Loss Injection UK 2026: Mounjaro vs Wegovy vs Saxenda

Three prescription weight loss injections are widely available through private pharmacies in the UK as of March 2026. All three belong to the GLP-1 receptor agonist class of medications and work by reducing appetite, slowing gastric emptying, and improving metabolic function.

• Mounjaro (tirzepatide) by Eli Lilly — a dual GIP/GLP-1 receptor agonist, injected once weekly. The newest and most effective option based on clinical trial data.
• Wegovy (semaglutide 2.4mg) by Novo Nordisk — a GLP-1 receptor agonist, injected once weekly. The first dedicated weight loss injection to receive NICE approval for NHS use.
• Saxenda (liraglutide 3mg) by Novo Nordisk — a GLP-1 receptor agonist, injected once daily. The oldest of the three, with smaller average weight loss but a longer safety track record.

All three require a prescription from a qualified prescriber (doctor or pharmacist independent prescriber) following a clinical assessment. None are available over the counter.

Clinical trial data provides clear rankings for average weight loss at maximum doses over comparable time periods:

• Mounjaro 15mg: 20.9% average body weight loss over 72 weeks (SURMOUNT-1). For a 100kg person: approximately 21kg lost.
• Wegovy 2.4mg: 14.9% average body weight loss over 68 weeks (STEP-1). For a 100kg person: approximately 15kg lost.
• Saxenda 3mg: 8.0% average body weight loss over 56 weeks (SCALE). For a 100kg person: approximately 8kg lost.

Mounjaro is the clear leader in clinical trial effectiveness, producing approximately 40% more weight loss than Wegovy and over 150% more than Saxenda. The dual GIP/GLP-1 mechanism of tirzepatide is believed to be the main reason for its superior efficacy. However, these are population averages. Individual responses vary considerably. Some people achieve excellent results on Saxenda and poor results on Mounjaro, and vice versa. If one medication is not working after 3-4 months, switching to an alternative is reasonable and often recommended by prescribers.

Cost is often the deciding factor for UK users. Here is how the three injections compare on price (cheapest available pharmacy prices, March 2026): Monthly cost at maintenance dose:

• Mounjaro (10-15mg): £160 – £320/month
• Wegovy (2.4mg): £190 – £300/month
• Saxenda (3mg): £120 – £200/month

Estimated 12-month treatment cost (cheapest pharmacies):

• Mounjaro: £1,800 – £2,200
• Wegovy: £2,000 – £2,600
• Saxenda: £1,440 – £2,400

Saxenda appears cheapest per month, but because it produces significantly less weight loss, the cost per kilogram lost tells a different story:

• Mounjaro: Approximately £90-105 per kg lost
• Wegovy: Approximately £135-175 per kg lost
• Saxenda: Approximately £180-300 per kg lost

On a cost-per-result basis, Mounjaro offers the best value. For live Mounjaro pricing across 60+ pharmacies, use our price comparison tool.

All three medications share similar gastrointestinal side effects because they all activate the GLP-1 receptor. The most common side effects are nausea, diarrhoea, constipation, and vomiting, typically occurring during dose titration and improving over time.

• Mounjaro: Nausea (15-24%), diarrhoea (12-17%), constipation (6-11%), vomiting (5-12%). Side effects are generally mild to moderate and most common during dose increases.
• Wegovy: Nausea (44%), diarrhoea (30%), vomiting (24%), constipation (24%). Higher reported rates than Mounjaro, though this may partly reflect different trial populations and reporting methods.
• Saxenda: Nausea (39%), diarrhoea (21%), constipation (19%), vomiting (16%). Daily injection also causes more frequent injection site reactions.

A key difference is injection frequency: Mounjaro and Wegovy are injected once per week, while Saxenda requires a daily injection. For many users, the convenience of weekly dosing is a significant advantage. All three medications carry rare but serious warnings for pancreatitis, gallbladder issues, and thyroid C-cell tumour risk (based on animal studies). These risks are considered low and are monitored during treatment.

Beyond effectiveness and cost, practical considerations matter when choosing a weight loss injection:

• Injection frequency: Mounjaro and Wegovy are once weekly. Saxenda is once daily. Weekly injections are more convenient and have higher adherence rates.
• NHS availability: Wegovy is the only one approved by NICE for NHS prescribing (in specialist weight management services, with long waiting lists). Mounjaro is NHS-available for type 2 diabetes but not yet for weight management. Saxenda has been available on the NHS through specialist services for longer but is being superseded by Wegovy.
• Stock reliability: Mounjaro has experienced periodic stock shortages, particularly for higher doses (12.5mg, 15mg). Wegovy supply has improved significantly since 2024 but shortages still occur. Saxenda generally has the most reliable supply.
• Pharmacy availability: All three are widely available from UK online pharmacies. Mounjaro and Wegovy are also offered by high-street pharmacies including Boots and Superdrug.
• Storage: All three require refrigeration (2-8°C). Once in use, Mounjaro and Wegovy pens can be kept at room temperature for up to 21 days, while Saxenda pens last 30 days at room temperature.

Based on the evidence available in March 2026, here is our summary:

• Best overall: Mounjaro. The highest average weight loss, competitive pricing, weekly injection, and growing availability make it the top choice for most people. It also offers the best cost-per-result value.
• Best alternative: Wegovy. Proven effective, well-established safety record, and the only option with NICE approval for NHS weight management. A strong choice if Mounjaro is unavailable or not tolerated.
• Budget option: Saxenda. The lowest monthly cost, but significantly less effective than Mounjaro or Wegovy. The daily injection schedule is also less convenient. Best suited for people who need a lower-cost entry point or who cannot tolerate the newer medications.

Regardless of which medication you choose, it should be prescribed by a qualified prescriber who assesses your health, monitors your progress, and adjusts your treatment as needed. Never buy weight loss injections from unregistered sellers or without a proper clinical assessment. To compare current Mounjaro prices across 60+ UK pharmacies, visit our price comparison page.