The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Mounjaro Vs Wegovy Side Effects — UK Prescribing Guide
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work differently and carry different side effect profiles. This guide compares the common and serious side effects of both medications in a UK NHS context, explains NHS eligibility through tier 3 specialist services, and helps you understand what clinical evidence shows about safety. Individual reactions vary significantly, and your GP or prescriber will assess which medication suits your health profile.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
Side Effects Comparison — Mounjaro Vs Wegovy
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously. In clinical trials, the most common side effects were gastrointestinal: nausea (25–32% of participants), diarrhoea (22–31%), vomiting (5–11%), and constipation (17–24%). Stomach pain affected around 13% of users. Most side effects are mild to moderate and tend to improve as your body adjusts to each dose.
Wegovy (semaglutide) is a single GLP-1 receptor agonist. In the STEP-1 trial, similar gastrointestinal side effects were reported: nausea (25–35%), diarrhoea (23%), vomiting (3–6%), and constipation (23%). The incidence of stomach pain was slightly lower than Mounjaro at around 11%. Because Wegovy uses only one hormone pathway, some prescribers observe slightly fewer injections site reactions, though this varies between individuals.
- Nausea and stomach upset: Both medications slow gastric emptying; effects typically diminish after 4–8 weeks on each dose
- Injection site reactions: Redness, bruising, or pain at injection site — more common with Mounjaro in clinical data
- Hair loss (alopecia): Reported by 3–5% on Mounjaro; less commonly reported with Wegovy
- Headache and fatigue: Both medications can cause tiredness and headaches, usually temporary
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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NHS Access, Eligibility & Safety Monitoring
Both Mounjaro and Wegovy are available through NHS tier 3 specialist weight management services following referral from your GP. NICE guidance approves tirzepatide (Mounjaro) for adults with a BMI of 30 or above, or 27–29.9 with weight-related comorbidities. Wegovy eligibility is similar. Access varies significantly by region — waiting times range from months to over a year in some areas. Mounjaro carries MHRA black triangle (▼) status, indicating enhanced monitoring; both medications require regular clinical review.
Serious side effects requiring urgent medical attention include severe stomach pain (possible pancreatitis), gallbladder complications, signs of thyroid cancer, or serious allergic reactions. Your prescriber will screen for contraindications before starting. The GLP-1 weight loss calculator on this page can help estimate potential weight loss at different doses, while the BMI calculator helps establish your baseline eligibility range. Either medication is not suitable during pregnancy, and oral contraceptives may have reduced absorption — ask your prescriber about barrier methods.
Choosing between Mounjaro and Wegovy often depends on individual response, availability through your NHS tier 3 service, and prescriber recommendation rather than side effect profile alone — both are well tolerated by most people. Mounjaro's dual mechanism may produce slightly more nausea initially but also slightly stronger appetite suppression. Wegovy has marginally better tolerability data in some studies but requires daily injections versus Mounjaro's once-weekly dosing. NHS specialist services will assess your medical history, comorbidities, and any previous medication reactions to recommend the most appropriate option. Most side effects are dose-dependent and manageable through slower dose escalation or temporary dose reduction. Always inform your prescriber immediately if you experience severe, persistent symptoms, changes in heart rate, or signs of pancreatitis.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Which causes more side effects, Mounjaro or Wegovy?
Clinical trials show very similar gastrointestinal side effect rates — both around 25–35% experience nausea. Mounjaro shows slightly more injection site reactions and hair loss; Wegovy shows slightly more tolerability. Individual responses vary significantly. Your prescriber will tailor the choice based on your health profile, not just side effect frequency alone.
Can I switch from Wegovy to Mounjaro on the NHS if I'm having side effects?
Yes, but this must be done under your NHS tier 3 service guidance. If side effects are severe, your prescriber may reduce your dose first, switch to the alternative medication, or consider pausing treatment. Always speak to your specialist rather than stopping abruptly — sudden discontinuation can cause rebound symptoms.
How long do side effects last on both medications?
Most gastrointestinal side effects (nausea, diarrhoea) peak in the first 1–2 weeks on each dose and improve significantly by week 4–8 as your body adjusts. Injection site reactions typically resolve within hours to days. Hair loss may develop gradually and persist for weeks after starting. Fatigue usually improves after the first month.
Are Mounjaro and Wegovy safe to use together?
No — these medications should never be used together. They work on overlapping hormone pathways; combining them would significantly increase side effects and safety risks. Your prescriber will recommend one medication, not both.
What's the difference between Ozempic and Wegovy side effects?
Both contain semaglutide but Ozempic is for type 2 diabetes (doses up to 2 mg) whereas Wegovy is licensed for weight management (up to 2.4 mg). At Wegovy's higher dose, gastrointestinal side effects are more pronounced. Ozempic is not NHS-approved for weight loss — using it for weight loss is off-label and not recommended on the NHS.
Ready to take control of your weight loss journey?
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