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 Pen Side Effects vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections used in the UK, but they work differently and can cause different side effects. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy activates GLP-1 only. Understanding the side effect profiles of each can help you and your prescriber decide which is more suitable for your individual health needs. This guide compares them factually based on clinical trial data and UK prescribing guidelines.
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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 — Clinical Trial Evidence
Mounjaro (tirzepatide) and Wegovy (semaglutide) share many common side effects because they both work on appetite and digestion. However, the frequency and severity can differ.
Most common side effects — both medications:
- Nausea and vomiting
- Diarrhoea and constipation
- Abdominal pain
- Indigestion
In the SURMOUNT-1 trial (Mounjaro), nausea affected over 25% of participants on the highest dose. In the STEP-1 trial (Wegovy), nausea was reported by approximately 25% at the maximum dose (2.4 mg). Both medications show that gastrointestinal side effects are most common during dose escalation and often improve over time. Hair loss (alopecia) was reported more frequently with Mounjaro in clinical trials than with Wegovy, though it remains relatively uncommon. Your prescriber will monitor side effects at each dose increase and adjust your treatment plan if needed.
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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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Choosing Between Them — Individual Suitability
Neither Mounjaro nor Wegovy is "better" — the right choice depends on your health profile, how you respond to treatment, and your prescriber's clinical judgment. Both require a prescription and clinical assessment before use. Your GP or online prescriber will consider factors including your weight loss goals, existing health conditions, other medications you take, and any previous side effects from similar medicines.
Calculate your personal BMI using our free BMI calculator above to understand your starting health category. Both medications are typically prescribed alongside a reduced-calorie diet and increased physical activity — medication alone is not effective without lifestyle changes. If you experience bothersome side effects on one medication, your prescriber may recommend switching to the other, though this requires a new assessment.
- 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 has fewer side effects, Mounjaro or Wegovy?
Both have similar common side effects (nausea, diarrhoea, stomach pain). Mounjaro may cause slightly more hair loss, while individual tolerance varies significantly. Your prescriber will assess which is better for your personal health profile. Neither is universally "safer" — it depends on your individual response.
Do side effects get better over time with either injection?
Yes, most gastrointestinal side effects improve after 4–8 weeks on each dose. Nausea and stomach upset are most common during dose escalation phases. Staying hydrated, eating smaller meals, and avoiding fatty foods can help. If side effects persist beyond 8 weeks, inform your prescriber.
Can I switch from Mounjaro to Wegovy if I get side effects?
Possibly, but it requires a new clinical assessment with your prescriber. Switching between medications is a clinical decision — you cannot simply change without medical guidance. There is no established conversion chart, so your prescriber will restart you at an appropriate Wegovy dose if they deem it suitable.
Are there serious side effects I should watch for with either medication?
Yes. Seek urgent medical attention for severe stomach pain (possible pancreatitis), serious allergic reactions (swelling of face/lips/throat), gallbladder problems, or rapid heart rate changes. These are rare but require immediate GP or A&E assessment. Always read the patient information leaflet before starting either medication.
How do Mounjaro and Wegovy differ in how they work?
Mounjaro activates two hormone pathways (GIP and GLP-1 agonist), while Wegovy activates GLP-1 only. This dual mechanism in Mounjaro may contribute to slightly better average weight loss in trials, but both are effective. The choice between them is clinical, not based on one being universally superior.
Ready to take control of your weight loss journey?
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