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
Why Have I Gained Weight on Mounjaro?
Weight gain while taking Mounjaro can happen, even though clinical trials show average weight loss of 15–21% body weight. This may occur if your dose is too low to be effective for you, if you've stopped following dietary changes, due to individual variation in how your body responds, or if an underlying health condition is affecting your metabolism. If you've gained weight on Mounjaro, it's important to discuss this with your prescriber — they may adjust your dose, review your lifestyle, or suggest an alternative weekly weight loss injection like Wegovy. Individual responses to GLP-1 and GIP/GLP-1 medications vary significantly.
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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.
Why Weight Gain Happens on Mounjaro — And When to Consider Switching
Weight gain on Mounjaro typically occurs for four main reasons. First, your current dose may be too low — starting doses of 2.5 mg are adjustment doses, and some people need higher doses (up to 15 mg) for effective appetite suppression. Second, lifestyle changes may have regressed — Mounjaro works alongside a reduced-calorie diet and physical activity, not instead of them. If calorie intake has increased or exercise has stopped, weight loss plateaus or reversal can occur. Third, individual response variation — not everyone responds equally to the same medication, even at the same dose. Clinical trials show averages, but some people experience less appetite suppression. Fourth, underlying health conditions like thyroid disorders, PCOS, or medication side effects from other prescriptions can interfere with weight loss. Your prescriber can investigate which factor applies to you and may recommend dose titration (gradual increase) over weeks, a switch to an alternative prescription weight loss injection, or referral back to a dietitian for dietary review.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Accessing Alternatives — Mounjaro vs Wegovy, and UK Cost Options
If Mounjaro isn't working for you, Wegovy (semaglutide) is an alternative GLP-1 weekly weight loss injection available in the UK. Clinical trials show similar average weight loss outcomes, though individual results vary. Both are prescription-only and available through NHS specialist weight management services or private online consultations. NHS access: Referral to tier 3 specialist services; availability and waiting times vary significantly by region. Private route: Online consultation with a UK-licensed prescriber costs approximately £20–£50; prescription dispensing via GPhC-registered pharmacy ranges from £129–£320 per month depending on dose. Neither route assures weight loss — your prescriber will assess suitability based on your health, current dose, and response history. Use our free GLP-1 weight loss calculator to explore projected outcomes at different doses with either medication.
- 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 would I gain weight instead of lose weight on Mounjaro?
Weight gain on Mounjaro usually means your dose is too low to suppress your appetite effectively, you've returned to higher calorie intake, or your body isn't responding as expected to the medication. Individual variation is normal — clinical trials show averages, not individual assures. Your prescriber can review your dose, lifestyle, and underlying health to identify the cause.
Will increasing my Mounjaro dose help if I've gained weight?
Possibly. If you're still on a low dose (under 10 mg), your prescriber may recommend gradual dose titration over weeks to find your effective dose. However, if you're already at 15 mg and still gaining weight, a different medication or deeper lifestyle review may be needed. Only your prescriber can recommend the right next step.
How much does it cost to switch to Wegovy privately in the UK?
Private Wegovy costs approximately £129–£320 per month, depending on your dose. An online consultation with a UK-licensed prescriber costs around £20–£50. Costs vary by provider. NHS access requires referral to a specialist weight management service, with no direct cost to you but potentially long waiting lists depending on your region.
Should I stop taking Mounjaro if I've gained weight, or switch immediately?
Never stop or switch without guidance from your prescriber. Stopping suddenly can affect your appetite control and weight further. Your prescriber may adjust your dose first, or they may recommend switching to Wegovy if clinically appropriate. Book an urgent review appointment with the prescriber who prescribed your Mounjaro.
Is weight gain on Mounjaro a sign the medication isn't working for me at all?
Not necessarily. Weight gain more often signals a dose that's too low, lifestyle changes, or reduced medication response over time rather than complete failure. Some people also experience fluctuations due to water retention or hormonal changes. A prescriber review is essential to distinguish between these possibilities and find the right solution for your situation.
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.