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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Do Mounjaro and Wegovy Both Cause Shivers?
Shivers and chills are not listed as common side effects of either Mounjaro (tirzepatide) or Wegovy (semaglutide) in UK prescribing information. However, both medications can cause nausea, diarrhoea, and other gastrointestinal effects that might make you feel generally unwell — which can sometimes be mistaken for shivers or chills. If you experience shivers while taking either medication, it's important to discuss this with your GP or prescriber, as it could indicate another cause. Both are prescription-only weight management injections available in the UK following clinical assessment.
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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.
How Mounjaro and Wegovy Compare in the Clinical Trials
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. In the SURMOUNT-1 clinical trial, participants on the 15 mg dose achieved an average weight loss of approximately 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. The most common side effects were gastrointestinal — nausea, diarrhoea, vomiting, and constipation — rather than shivers or chills.
Wegovy (semaglutide) is a GLP-1 receptor agonist only. In the STEP-1 clinical trial, participants on the 2.4 mg dose achieved an average weight loss of approximately 14.9% of their body weight over 68 weeks. Like Mounjaro, the most frequently reported side effects were nausea, diarrhoea, vomiting, and constipation. Neither medication listed shivers as a very common or common side effect in trial data.
Both medications work by affecting appetite and food intake, but Mounjaro's dual mechanism may produce slightly different effects. Individual responses vary considerably, and what one person experiences may differ from another.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Which Medication Might Be Right for You?
Neither Mounjaro nor Wegovy is inherently "better" — suitability depends on your individual health profile, medical history, and how your body responds. Both are prescription-only and require a clinical assessment from a UK-licensed prescriber before being prescribed. Your GP or an online prescriber will assess factors including your BMI, any weight-related health conditions, other medications you take, and your medical history.
If you experience unusual symptoms like shivers while on either medication, contact your prescriber immediately — this could indicate a separate health issue. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to check your current health weight category. Both tools provide context for discussions with your healthcare provider about which medication might suit you.
- 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.
Are shivers a common side effect of Mounjaro?
No. Shivers and chills are not listed as common or very common side effects of Mounjaro in UK prescribing information. The most frequent side effects are gastrointestinal — nausea, diarrhoea, vomiting, and constipation. If you experience shivers, contact your GP or prescriber as this could indicate another health issue.
Does Wegovy cause shivers or chills?
Shivers are not reported as a common side effect of Wegovy either. Like Mounjaro, Wegovy's most frequent side effects are nausea, diarrhoea, vomiting, and constipation. Unusual symptoms including shivers should be reported to your prescriber.
What is the main difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whereas Wegovy (semaglutide) is a GLP-1 agonist only. This dual mechanism in Mounjaro may produce different effects. In clinical trials, Mounjaro showed slightly higher average weight loss (approximately 20.9% vs 14.9%), but individual results vary significantly.
Can I switch between Mounjaro and Wegovy if I get side effects?
Any switch between weight loss medications must be managed by your prescriber. They will assess your individual circumstances and decide whether switching is appropriate. Never stop or change medication without medical guidance. Contact your GP or prescriber if side effects are affecting you.
How do I access Mounjaro or Wegovy in the UK?
Both medications are prescription-only and available through NHS specialist weight management services (tier 3/4) or private online prescribing services. You'll need a clinical assessment before either can be prescribed. NHS access varies by region; private access is available without NHS referral through GPhC-registered pharmacies.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.