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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Back Ache on Mounjaro: What You Need to Know
Back pain and muscle aches are not commonly listed side effects of Mounjaro (tirzepatide) in clinical trials, but some users report discomfort during weight loss. This may relate to rapid weight reduction, changes in posture, or increased physical activity rather than the medication itself. If you experience back ache while taking Mounjaro, it's important to discuss this with your GP or prescriber, as they can assess whether it's medication-related or linked to other factors. Using our free BMI calculator, you can track how your weight changes relate to any symptoms you experience.
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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.
Mounjaro vs Wegovy: Side Effect and Mechanism Differences
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work differently. Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways, while Wegovy is a GLP-1 agonist only. This difference affects how they work in the body and potentially which side effects users experience.
In clinical trials (SURMOUNT for Mounjaro, STEP for Wegovy), the most common side effects for both medications were gastrointestinal: nausea, diarrhoea, vomiting, and constipation. Musculoskeletal pain including back ache was not a primary reported side effect in either trial. However, gradual weight management itself can place stress on joints and muscles, particularly if physical activity increases. Some users also experience postural changes during significant weight reduction, which may contribute to back discomfort. Your prescriber can help you manage any side effects and determine whether they relate to the medication or weight loss process.
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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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Is Back Ache More Common on Mounjaro or Wegovy?
Neither medication lists back ache as a common or very common side effect according to UK prescribing information. However, both can cause general muscle and joint discomfort indirectly through gradual weight management. Mounjaro's dual mechanism may theoretically affect metabolism differently than Wegovy, but clinical trial data does not show a significant difference in musculoskeletal complaints between the two.
If you develop back pain while on either medication, contact your GP or prescriber immediately. They will assess whether the pain is related to the injection, your changing weight, increased exercise, or an underlying condition. Weight loss itself can sometimes improve back pain (reducing spinal load), but rapid loss may temporarily increase discomfort. Your prescriber may adjust your dose, recommend physiotherapy, or suggest other support. Use our GLP-1 weight loss calculator to track your progress and discuss any symptoms with your healthcare team at each review.
- 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 back ache a side effect of Mounjaro?
Back ache is not listed as a common side effect of Mounjaro in clinical trials or UK prescribing information. However, gradual weight management and postural changes during weight reduction can contribute to muscle or joint discomfort. If you experience back pain while taking Mounjaro, discuss it with your GP or prescriber to rule out other causes and explore management options.
Can Wegovy cause back pain?
Like Mounjaro, Wegovy does not list back pain as a common side effect. However, significant weight loss and increased physical activity can place temporary stress on joints and muscles. Back discomfort during weight loss is usually indirect and manageable. Always report any new or persistent pain to your prescriber.
Why might my back hurt during weight loss on GLP-1 medication?
Several factors may contribute: gradual weight management can alter posture and spinal load; increased physical activity as energy improves; mobilisation of stored areas of tension; or rarely, an unrelated condition emerging coincidentally. Your GP can assess your specific situation and may recommend physiotherapy, exercise modification, or pain relief.
Should I stop Mounjaro if I develop back ache?
Do not stop your medication without medical guidance. Contact your GP or prescriber first to discuss your symptoms. They will determine whether the pain relates to the medication, weight loss, or another cause, and decide whether to adjust your dose, pause treatment temporarily, or recommend additional support like physiotherapy.
Is Mounjaro or Wegovy better if I have back pain?
There is no clear evidence that one medication causes more back pain than the other. The choice between Mounjaro and Wegovy depends on your BMI, health conditions, prescriber preference, and suitability following clinical assessment. Both require a prescription and individual evaluation. Discuss any existing back pain with your prescriber before starting either medication.
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