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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Can't Eat on Mounjaro vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections that can significantly reduce appetite — which is exactly how they help with weight loss. However, the reasons you may struggle to eat on each medication differ. Mounjaro uses a dual GIP and GLP-1 mechanism, whilst Wegovy works through GLP-1 alone. Understanding these differences can help you and your GP or prescriber decide which might suit you better. Individual responses vary considerably, and both require clinical assessment before prescription.
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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 Affect Appetite — The Mechanism Difference
Mounjaro (tirzepatide) activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action may amplify appetite suppression more than GLP-1 alone, meaning some users report feeling fuller faster and losing interest in food more quickly. The SURMOUNT clinical trials showed that participants on Mounjaro experienced reduced appetite as a primary effect, with average weight loss ranging from 15% to 20.9% of body weight depending on dose.
Wegovy (semaglutide) activates GLP-1 receptors only, which triggers satiety signals in the brain and slows gastric emptying (how quickly food leaves your stomach). The STEP-1 trial demonstrated average weight loss of approximately 14.9% body weight. Many users report nausea and reduced appetite, though the appetite suppression mechanism is single-pathway rather than dual.
Because Mounjaro's dual mechanism may produce stronger appetite suppression, some users report greater difficulty eating on Mounjaro compared to Wegovy — though individual variation is significant.
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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.
Side Effects and Eating Difficulty — Which Is More Likely?
Both medications list nausea, vomiting, and loss of appetite as very common side effects. On Mounjaro, nausea and appetite loss tend to appear within days of starting and during dose escalation, typically improving after 1–2 weeks at each new dose level. Some users report that eating becomes genuinely difficult because food is unappealing or causes nausea; others find they simply feel full on very small portions.
On Wegovy, nausea is similarly very common, particularly in the first 2–4 weeks of treatment and during dose increases. Appetite reduction is usually the goal rather than a side effect — it's central to how the medication works.
If you find eating difficult on either medication, inform your prescriber immediately. Persistent vomiting, inability to keep food down, or severe abdominal pain may indicate pancreatitis and requires urgent medical attention. Your prescriber may slow your dose escalation or adjust your plan. Use our free GLP-1 weight loss calculator to understand what to expect with either medication, and track side effects using our progress tracker.
- 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.
Can I switch from Mounjaro to Wegovy if I can't eat on Mounjaro?
Yes, switching is possible and is managed by your prescriber. Mounjaro's dual mechanism may cause stronger appetite suppression in some individuals. An approximate equivalence exists (Mounjaro 5mg ≈ Wegovy 0.5mg), but any switch requires a prescriber's guidance and must be done carefully to avoid dose errors. Your prescriber will assess whether switching is appropriate for you.
Will the "can't eat" feeling go away on either medication?
For most users, appetite suppression lessens slightly over time, though it remains the primary benefit. Nausea and vomiting typically improve after 1–2 weeks at each dose level. If difficulty eating persists or worsens beyond the first few weeks of a new dose, contact your GP or prescriber — it may indicate intolerance or need for dose adjustment.
Is it dangerous to not eat much on Mounjaro or Wegovy?
No, if you're eating enough to meet your nutritional needs. However, if you're unable to keep food down, experiencing severe nausea, or eating significantly less than 800 kcal daily unintentionally, inform your prescriber. They'll assess whether to pause escalation, adjust your dose, or switch medications. Malnutrition is a genuine concern if appetite suppression is extreme.
Which medication causes less nausea — Mounjaro or Wegovy?
Clinical trials show both cause nausea in over 1 in 10 users. Mounjaro's dual mechanism may cause slightly more appetite suppression overall, but individual responses vary enormously. One person may tolerate Mounjaro well whilst another experiences severe nausea; the opposite can be true for Wegovy. Your prescriber cannot predict which you'll tolerate better — it depends on your biology.
How do I check if Mounjaro or Wegovy is right for me?
Both require a clinical assessment with a UK-licensed prescriber to check your eligibility and medical history. You must have a BMI of 30 or above, or 27–29.9 with a weight-related health condition (type 2 diabetes, high blood pressure, heart disease, etc.). Your prescriber will discuss both options, side effects, and which may suit you best. Use our BMI calculator to check your current BMI category.
Ready to take control of your weight loss journey?
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