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 Feeling Faint vs Wegovy — What's the Difference?
Feeling faint or dizzy on weight loss injections is a known side effect that can occur with both Mounjaro and Wegovy, though the underlying causes differ slightly. Both medications are prescription-only GLP-1 or dual-mechanism injections licensed in the UK for weight management, but they work differently and affect individuals in varied ways. Understanding how each medication may impact you—and what the clinical trial data shows—can help you and your prescriber decide which is right for your situation. Use our free GLP-1 weight loss calculator to explore potential outcomes with either medication.
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Calculate Your BMI Instantly
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 Differ in Mechanism and Side Effects
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways. Wegovy (semaglutide) is a GLP-1-only agonist. This difference in mechanism can influence side effect profiles.
Feeling faint can occur because both medications slow gastric emptying (how quickly food leaves your stomach) and may lower blood pressure slightly, particularly in the first few weeks. In clinical trials:
- Mounjaro (SURMOUNT-1 trial): Dizziness was reported in common side effects; fainting is rare but documented
- Wegovy (STEP trials): Dizziness and lightheadedness were similarly common; severe fainting was uncommon
The dual mechanism of Mounjaro means it may cause slightly different patterns of nausea and appetite suppression compared to Wegovy, which could indirectly affect blood pressure or fluid intake—both factors in dizziness. Individual responses vary considerably.
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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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Which Medication Might Suit You Better? A Prescriber Decision
Feeling faint on either medication doesn't automatically mean you need to stop—often it improves as your body adjusts over weeks. However, your prescriber must assess your individual risk factors. If you have a history of low blood pressure, heart rhythm issues, or dehydration, one medication may be safer than the other.
Both medications require a clinical assessment before prescription. Your GP or private prescriber will review your medical history, current medications, and any previous reactions to GLP-1 medications. If you're experiencing fainting or severe dizziness on either Mounjaro or Wegovy, contact your prescriber immediately—do not adjust your dose alone. They may recommend dose reduction, slower escalation, or switching to the alternative medication if appropriate.
- 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.
Why do both Mounjaro and Wegovy cause feeling faint?
Both slow stomach emptying and may slightly lower blood pressure. This can reduce blood flow to the brain temporarily, especially if you're dehydrated or haven't eaten enough. Fainting is rare but dizziness is more common. It often improves as your body adapts over the first few weeks of treatment.
Is fainting more common on Mounjaro or Wegovy?
Clinical trial data shows dizziness and lightheadedness occur at similar rates with both medications. Fainting specifically is uncommon with either. Individual responses vary based on your baseline blood pressure, kidney function, and how quickly your dose increases. Your prescriber can help minimise risk.
What should I do if I feel faint on Mounjaro or Wegovy?
Stop what you're doing and sit or lie down immediately. Drink water if possible. Contact your GP or prescriber the same day. Do not drive or operate machinery until you've spoken to a healthcare professional. Never adjust your dose without guidance—your prescriber may slow your escalation or explore alternatives.
Can I switch from Mounjaro to Wegovy if I'm feeling faint?
Yes, switching is possible and your prescriber may recommend it if fainting is severe. There is no official conversion chart between the two medications—your prescriber will establish a new dose schedule based on your clinical response. Always make any switch under medical supervision.
How can I reduce the risk of feeling faint on weight loss injections?
Stay hydrated throughout the day, eat small regular meals even if appetite is reduced, avoid sudden position changes, and inform your prescriber of any blood pressure medications you take. Ensure adequate salt intake unless medically advised otherwise. Report any fainting to your GP immediately—it may indicate a need for dose adjustment.
Ready to take control of your weight loss journey?
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