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
Can Mounjaro Stop Working Over Time?
Yes, some people experience reduced weight loss effectiveness with Mounjaro over time — a phenomenon sometimes called tolerance. However, this doesn't mean the medication has completely stopped working. Understanding why this happens, and what options are available, can help you maintain your results. Whether you're considering Mounjaro through NHS specialist services or private prescription in the UK, knowing how to manage effectiveness is important for long-term success.
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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 Mounjaro May Become Less Effective
When you first start Mounjaro (tirzepatide), your body responds significantly to the medication. Over time, your body adapts to the presence of the GIP and GLP-1 receptor agonist — this is called tolerance. This is a normal biological response, not a failure of the medication. Several factors contribute to reduced effectiveness: your body weight decreases, meaning a smaller absolute dose reaches your system; appetite suppression effects may naturally plateau as your body adjusts; and your baseline hunger and metabolism shift as weight loss progresses. The dose titration schedule (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, 15 mg) is designed to help manage this by increasing the dose every 4 weeks if needed. If you're not seeing continued weight loss, your prescriber may recommend escalating to the next dose rather than assuming the medication has stopped working entirely.
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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.
What to Do If Mounjaro Feels Less Effective
If you feel Mounjaro is becoming less effective, discuss this with your prescriber at your next review appointment. They may recommend dose escalation — moving to the next available dose — if you haven't reached the maximum 15 mg. Your prescriber will also assess whether lifestyle factors have changed: reduced calorie deficit, increased food intake, or decreased physical activity will naturally reduce weight loss, regardless of medication. Using our free GLP-1 weight loss calculator can help you track your expected results and spot changes in your progress. Additionally, your prescriber may consider whether a break from the medication (washout period) might help reset tolerance, though this is individual and requires clinical guidance. Weight loss plateaus are common and don't mean failure — adjusting dose or lifestyle often restarts progress.
- 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.
Does Mounjaro stop working completely?
No, Mounjaro doesn't stop working completely, but its effectiveness may plateau as your body adapts to the medication. This is called tolerance. Most people can address this through dose escalation, lifestyle adjustment, or both. Your prescriber can assess whether a dose increase is appropriate for you.
How long does it take for Mounjaro to stop being effective?
There's no fixed timeline — some people experience plateau effects after 3–6 months, while others maintain steady weight loss for longer. The timing depends on your starting weight, metabolism, lifestyle, and current dose. Regular check-ins with your prescriber help identify when adjustment might help.
Can you increase your Mounjaro dose if it stops working?
Yes. Mounjaro dosing starts at 2.5 mg and goes up to a maximum of 15 mg once weekly. If weight loss slows and you're not yet at maximum dose, your prescriber may recommend escalating. Each dose increase requires a minimum 4-week period before the next increase.
Is Mounjaro vs Wegovy different in terms of tolerance?
Both Mounjaro (tirzepatide, a dual GIP+GLP-1 agonist) and Wegovy (semaglutide, a GLP-1 agonist only) can experience tolerance over time. Mounjaro's dual mechanism may offer different effectiveness patterns, but individual responses vary. Your prescriber can help assess which is more suitable for you.
What's the difference between Mounjaro stopping working and needing a dose increase?
If Mounjaro "stops working," it usually means your body has adapted and weight loss has plateaued — not that the medication is inactive. A dose increase often restarts results. True medication failure is rare. If you're unsure, your GP or prescriber can help clarify whether adjustment or other factors are at play.
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.