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: Lower Back Pain and Side Effects
Lower back pain is not a recognised side effect of either Mounjaro (tirzepatide) or Wegovy (semaglutide) in clinical trials. Both are prescription-only GLP-1 (and dual GIP/GLP-1) weight management injections used in the UK. However, weight loss itself can sometimes ease lower back pain by reducing strain on the spine. This guide compares how each medication works and how your prescriber assesses which might suit your individual health needs.
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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.
How Mounjaro and Wegovy Work — Key Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways to help regulate appetite and support feeling fuller after meals. In the SURMOUNT-1 clinical trial, participants taking the maximum 15 mg dose lost an average of approximately 20.9% of their body weight over 72 weeks.
Wegovy (semaglutide) is a single GLP-1 receptor agonist, activating one hormone pathway. In the STEP-1 trial, participants on the maximum 2.4 mg dose lost an average of approximately 14.9% of body weight over 68 weeks.
- Both are once-weekly injections available by prescription in the UK
- Both are prescribed for BMI 30+ or BMI 27–29.9 with weight-related health conditions
- Both require ongoing clinical assessment and dose titration over several weeks
- Weight loss from either medication may help reduce lower back strain — a common benefit of losing excess weight
Neither medication lists lower back pain as a reported side effect. Any existing back pain should be discussed with your GP before starting either treatment.
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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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Which Medication Is Right for You?
Your prescriber will assess your individual health, medical history, and eligibility to determine whether Mounjaro or Wegovy is more suitable. Both require a UK-registered prescriber and clinical assessment — they are not interchangeable without medical guidance. You can use our free GLP-1 weight loss calculator to explore projected weight loss at different doses, and our BMI calculator to check your current position against NHS guidelines. This information supports your conversation with your prescriber.
Important: If you have lower back pain or any existing musculoskeletal condition, mention this when seeking assessment. Sustainable weight loss through either medication, combined with physical activity and dietary changes, often improves back health over time. Individual results vary significantly — always discuss your specific health needs with your GP or prescriber.
- 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.
Is lower back pain a side effect of Mounjaro?
Lower back pain is not listed as a recognised side effect of Mounjaro in clinical trials or product guidance. Common side effects include nausea, diarrhoea, constipation, and stomach pain. If you experience new or worsening back pain while taking Mounjaro, contact your GP or prescriber for assessment.
Can Wegovy cause lower back pain?
Wegovy does not list lower back pain as a side effect in clinical trial data. Common side effects are gastrointestinal (nausea, diarrhoea, vomiting, constipation) and injection site reactions. Any new pain should be reported to your prescriber for proper evaluation.
Can weight loss from Mounjaro or Wegovy help with back pain?
Yes, weight loss can help reduce lower back pain by decreasing strain on the spine and joints. Both medications support weight loss when combined with a reduced-calorie diet and increased physical activity. Many people experience improved back health as weight decreases, though individual outcomes vary.
Which is more effective — Mounjaro or Wegovy?
Based on clinical trial data, Mounjaro (tirzepatide) showed slightly higher average weight loss (~20.9% at maximum dose) compared to Wegovy (~14.9% at maximum dose). However, individual results vary significantly, and effectiveness depends on your personal response, adherence, and lifestyle changes. Your prescriber will assess which suits you best.
How do I access Mounjaro or Wegovy in the UK?
Both medications require a prescription from a UK-licensed prescriber. You can access them through NHS specialist tier 3 weight management services (with referral from your GP) or through private online prescribing services following a clinical assessment. Availability and waiting times vary by region and service.
Ready to take control of your weight loss journey?
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