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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Why Does Mounjaro Make You Feel Light Headed?
Feeling light headed whilst taking Mounjaro is a recognised side effect that some UK patients experience. Dizziness can occur for several reasons: rapid changes in blood sugar levels, dehydration from nausea and vomiting, lower calorie intake due to reduced appetite, or changes in blood pressure. Most cases are mild and temporary, but it's important to understand what's happening and when to seek advice from your GP. If you're taking Mounjaro through an NHS tier 3 weight management service or via private prescription, knowing how to manage light-headedness safely will help you stay on treatment.
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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.
Common Causes of Light Headedness on Mounjaro
Light headedness on Mounjaro typically occurs for one of several reasons:
- Rapid appetite suppression: When Mounjaro reduces your hunger significantly, you may eat much less than usual. This sudden calorie drop can cause dizziness, fatigue, and light-headedness, especially in the first few weeks of treatment.
- Dehydration: Nausea and vomiting are common Mounjaro side effects. If you're losing fluids and not replacing them adequately, dehydration can cause dizziness and light-headedness.
- Blood sugar changes: If you're taking Mounjaro alongside diabetes medication (such as insulin or sulphonylureas), your blood sugar may drop more rapidly, causing dizziness.
- Blood pressure drop: Some patients experience lower blood pressure on tirzepatide, which can trigger lightheadedness, especially when standing quickly.
- Dose escalation: Light-headedness is more common when increasing your Mounjaro dose or during the first 4 weeks on a new dose. Most symptoms improve as your body adjusts.
Most cases resolve within a few days to a week as your body adapts. However, if dizziness is severe or persistent, contact your GP immediately.
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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.
When to Contact Your NHS GP
Contact your GP or NHS weight management service urgently if you experience:
- Severe or worsening dizziness that prevents you from standing or walking safely
- Light-headedness combined with chest pain, shortness of breath, or fainting
- Persistent dizziness lasting more than a week despite hydration and dietary adjustments
- Dizziness accompanied by severe abdominal pain (possible pancreatitis)
- Signs of severe dehydration: dark urine, extreme thirst, dry mouth, rapid heartbeat
If you're prescribed Mounjaro through the NHS, your tier 3 service should provide contact information for urgent queries. If accessing Mounjaro privately, contact your prescriber immediately. Never stop taking Mounjaro without medical guidance—always discuss side effects with your healthcare provider first.
- 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 light-headedness a common side effect of Mounjaro in the UK?
Light-headedness is a recognised but not extremely common side effect of Mounjaro. It's listed as an uncommon side effect affecting up to 1 in 100 patients in clinical trials. However, when combined with nausea (which is very common), dehydration, and reduced food intake, dizziness becomes more likely during the first few weeks of treatment.
How can I prevent feeling light headed when taking Mounjaro?
Stay hydrated by drinking water consistently throughout the day, even if you're not thirsty. Eat regular small meals to maintain stable blood sugar—don't skip meals despite reduced appetite. Stand up slowly from sitting or lying down to avoid postural dizziness. Avoid alcohol, which increases dehydration. If you're on diabetes medication alongside Mounjaro, ask your GP about dose adjustments to prevent low blood sugar.
Should I stop taking Mounjaro if I feel dizzy?
Do not stop Mounjaro without speaking to your GP or prescriber first. Most light-headedness resolves within days as your body adjusts to the medication. However, if dizziness is severe, persistent, or accompanied by other symptoms like chest pain or fainting, contact your healthcare provider urgently. They can advise whether to continue, pause, or adjust your dose.
Can Mounjaro dose cause light-headedness to get worse?
Yes. Light-headedness can worsen when increasing your Mounjaro dose (e.g. from 5 mg to 7.5 mg), because your body is adjusting to a stronger appetite suppressant effect. This typically improves after 1–2 weeks. If dizziness is severe during dose escalation, contact your GP—they may recommend staying on your current dose longer before increasing.
Is light-headedness different between Mounjaro and Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) can cause dizziness, but Mounjaro's dual GIP and GLP-1 mechanism may trigger slightly different patterns. Mounjaro users sometimes report dizziness linked to faster appetite suppression. Wegovy users report similar side effects overall. If you're considering switching between these medications, discuss side effect differences with your prescriber—they can assess which is more suitable for you.
Ready to take control of your weight loss journey?
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