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
Free tracker included with every download this week
Does Mounjaro Make You Thirsty? A Comparison with Wegovy
Thirst is not listed as a common side effect of Mounjaro in clinical trials, though some users report increased thirst. When comparing Mounjaro (tirzepatide) with Wegovy (semaglutide), both GLP-1 based medications work slightly differently and may cause varying side effect experiences. Understanding what to expect helps you prepare for treatment. If you're considering either medication, our free BMI calculator can help confirm your eligibility, and a prescriber will assess which option suits your individual health needs during a clinical assessment.
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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.
Mounjaro vs Wegovy: Thirst and Dehydration Risk
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways. In the SURMOUNT-1 clinical trial, thirst was not recorded as a common or very common side effect. However, because Mounjaro slows gastric emptying (how quickly food leaves the stomach) and may reduce overall fluid intake through decreased appetite, some users report experiencing increased thirst indirectly.
Wegovy (semaglutide) is a GLP-1 receptor agonist only—a single mechanism. Similarly, thirst is not listed as a common side effect in STEP trial data. Both medications can cause nausea, vomiting, and diarrhoea, which may contribute to dehydration if fluid intake isn't managed carefully.
- Mounjaro: dual GIP + GLP-1 mechanism; thirst not commonly reported
- Wegovy: GLP-1 only; thirst also not commonly reported
- Both medications: dehydration risk exists if you don't drink enough water, especially with gastrointestinal side effects
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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.
Managing Hydration and Monitoring Side Effects
If you're prescribed either Mounjaro or Wegovy, staying hydrated is essential. Drink plenty of water throughout the day, even if you don't feel particularly thirsty. This is especially important during the first few weeks of treatment and dose titration, when your body is adjusting. If you experience persistent thirst, dry mouth, or signs of dehydration (dark urine, dizziness, extreme fatigue), contact your GP or prescriber immediately. Use our free GLP-1 weight loss calculator to understand potential outcomes, and keep your prescriber updated on any side effects during your treatment journey.
- 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 increased thirst a side effect of Mounjaro?
Thirst is not listed as a common side effect in Mounjaro clinical trials. However, some users report it indirectly due to reduced overall fluid intake from decreased appetite, or from gastrointestinal side effects like nausea or diarrhoea. Always mention any new symptoms to your prescriber.
Does Wegovy cause thirst like Mounjaro does?
No—thirst is not a reported common side effect of either medication in clinical trials. Both medications can cause dehydration indirectly through nausea, vomiting, or diarrhoea. Staying hydrated by drinking plenty of water is recommended for both treatments.
How does Mounjaro's dual GIP+GLP-1 mechanism differ from Wegovy's GLP-1 only mechanism?
Mounjaro activates two hormone pathways (GIP and GLP-1), while Wegovy activates GLP-1 only. This dual action may produce slightly different side effect profiles, though both are generally well-tolerated. Individual responses vary significantly.
What should I do if I experience unusual thirst on Mounjaro or Wegovy?
Contact your GP or prescriber if you develop persistent thirst, dry mouth, or signs of dehydration. Ensure you're drinking adequate water daily—aim for 6–8 glasses. This is especially important during dose titration phases.
How can I tell if I'm dehydrated while taking weight loss injections?
Signs of dehydration include dark or concentrated urine, dizziness, extreme fatigue, headaches, and dry mouth. If you experience these symptoms, increase your water intake and contact your prescriber. Dehydration can affect how well your medication works.
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.