Mounjaro (tirzepatide) and Wegovy (semaglutide) are the two most effective prescription weight loss medications currently available in the UK. Both are weekly injections. Both work through GLP-1 receptor activation. Both are genuinely effective. But they differ in mechanism, efficacy, dosing, side effect profiles, and cost in ways that matter for choosing between them.
Mechanism of action — why the difference matters
| Feature | Mounjaro (tirzepatide) | Wegovy (semaglutide) |
|---|---|---|
| Drug class | Dual GLP-1 / GIP receptor agonist | GLP-1 receptor agonist |
| Receptor targets | GLP-1 + GIP (dual) | GLP-1 only |
| Mechanism | GLP-1: appetite, gastric emptying, insulin. GIP: fat metabolism, energy balance, enhanced insulin secretion | Appetite suppression, gastric emptying, insulin secretion |
| Biological effect | Broader metabolic effect; GIP addition appears to enhance fat oxidation specifically | Strong appetite suppression; well-established mechanism |
| Clinical significance | The dual mechanism explains why tirzepatide consistently outperforms semaglutide in head-to-head comparisons | Very effective but narrower mechanism than tirzepatide |
Efficacy comparison — the clinical evidence
| Measure | Mounjaro (SURMOUNT-1, 15mg) | Wegovy (STEP-1, 2.4mg) |
|---|---|---|
| Average weight loss | 22.5% body weight | 14.9% body weight |
| Participants losing >15% body weight | 69.9% | 32.0% |
| Participants losing >20% body weight | 56.8% | 12.4% |
| Duration | 72 weeks | 68 weeks |
| Placebo comparison | +2.4% (placebo) | +2.4% (placebo) |
These are not directly comparable studies (different populations, protocols), but the gap is consistent across multiple analyses: Mounjaro produces approximately 50% more weight loss than Wegovy at maximum doses. The SURMOUNT-MMO head-to-head trial (2024) confirmed tirzepatide’s superiority.
Side effects — comparison
| Side effect | Mounjaro | Wegovy | Notes |
|---|---|---|---|
| Nausea | ~40–45% at dose increases | ~44% overall | Similar frequency; both improve after 2–4 weeks at each dose |
| Constipation | 12–17% | 24% | More common on Wegovy |
| Diarrhoea | 8–12% | 29% | Significantly more common on Wegovy |
| Hair loss | 4.9–7.1% (MHRA common) | 3% (Wegovy studies) | Both medications — primarily telogen effluvium |
| Injection site reactions | 3–8% | 14–26% | More common on Wegovy |
| Pancreatitis (serious, rare) | Very rare | Very rare | Both carry this black box warning |
Cost comparison UK (2026)
| Medication | Private prescription cost | NHS availability | Notes |
|---|---|---|---|
| Mounjaro (tirzepatide) | £100–160/month (typical private) | Limited — NHS specialist services for T2DM | More widely available privately in UK |
| Wegovy (semaglutide 2.4mg) | £200–250/month (typical private) | ✅ NICE approved for NHS (BMI ≥35 + comorbidity) | NHS pathway exists but waiting lists are long; private is cheaper than Wegovy |
The cost differential is significant: Mounjaro is typically £40–100 cheaper per month than Wegovy through private prescriptions in the UK. For a 12-month treatment course, this represents £480–£1,200 in savings. Combined with greater average weight loss, Mounjaro provides better value per kilogram lost.
UK availability and access
| Pathway | Mounjaro | Wegovy |
|---|---|---|
| Private prescription | ✅ Widely available through UK weight management clinics and online services | ✅ Available but more expensive |
| NHS — primary care | Limited (mainly type 2 diabetes indication) | Limited (NICE-approved pathways rolling out slowly) |
| NHS — specialist weight services | Available through some tier 3 obesity services | ✅ NICE recommended; rolling out through NHS England |
| Online weight management services | ✅ Yes — multiple UK services (Numan, Juniper, Voy) | ✅ Yes |
Which should you choose?
| Choose Mounjaro if: | Choose Wegovy if: |
|---|---|
| You want the most effective option available and cost is similar | You qualify for NHS Wegovy pathway and cost is the priority |
| You have type 2 diabetes or insulin resistance alongside obesity | You have previously responded well to semaglutide (Ozempic) |
| Cost is a significant factor (Mounjaro is typically cheaper privately) | Your prescriber specifically recommends Wegovy for your health profile |
| You have not responded adequately to semaglutide | The NHS Wegovy pathway is genuinely accessible to you |
Frequently Asked Questions
Is Mounjaro more effective than Wegovy?
Yes — clinical trial data consistently shows Mounjaro produces approximately 50% more weight loss than Wegovy at maximum doses. The SURMOUNT-MMO head-to-head trial confirmed tirzepatide’s superiority. The dual GLP-1/GIP mechanism of tirzepatide appears to produce stronger appetite suppression and greater fat loss.
Is Wegovy available on the NHS?
Yes — NICE approved Wegovy (semaglutide 2.4mg) for NHS use in adults with BMI ≥35 plus at least one weight-related comorbidity, or BMI 30–35 in specific high-risk groups. NHS rollout is gradual and waiting lists exist in most areas.
Can I switch from Wegovy to Mounjaro?
Yes — many people who have reached maximum benefit on Wegovy switch to Mounjaro for additional weight loss. The switch requires prescriber guidance on timing and dose titration. Most people experience continued weight loss after switching.
Which has fewer side effects — Mounjaro or Wegovy?
Head-to-head, Mounjaro tends to show lower rates of gastrointestinal side effects (particularly diarrhoea and injection site reactions) than Wegovy in clinical populations. Both carry similar nausea profiles. Individual response varies — some people tolerate one medication significantly better than the other.
How do I get Mounjaro prescribed in the UK?
Through a private weight management clinic or online weight management service (Numan, Juniper, Voy, or equivalent). Requires a consultation with a prescriber, medical history review, and BMI assessment. NHS access is currently limited.
Related: GLP-1 Side Effects UK | How to Lose Weight on Mounjaro UK
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