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 Mounjaro Might Not Be Working For You
Mounjaro (tirzepatide) can be an effective weight loss support tool, but it doesn't work the same way for everyone. If you're on Mounjaro and not seeing results after several weeks, there are usually identifiable reasons—from insufficient dosing to lifestyle factors, medical conditions, or simply needing more time. This guide explains the most common reasons why Mounjaro may not be delivering the results you expected, and what you can discuss with your prescriber or GP. Remember that individual results vary significantly, and what works depends on your personal health circumstances.
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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 Reasons Why Mounjaro May Not Be Working
Insufficient dose or early timeline: Mounjaro starts at 2.5 mg and increases every 4 weeks up to 15 mg. Many people don't see significant effects until they reach a therapeutic dose (usually 10 mg or higher). If you're still in the early titration phase, your body may not yet be receiving enough medication to suppress appetite effectively.
Not following a reduced-calorie diet: Mounjaro works alongside lifestyle changes, not instead of them. Clinical trials showed weight loss only occurred when participants combined the injection with a reduced-calorie diet and increased physical activity. Without a calorie deficit, even the maximum dose may not produce meaningful results.
Medical factors: Certain conditions affect how well GLP-1 medications work—including severe insulin resistance, uncontrolled type 2 diabetes, gastroparesis, or medications that interfere with absorption (like oral contraceptives). Your prescriber needs to know about all your health conditions and current medications to assess whether Mounjaro is suitable or if adjustments are needed.
Dose interactions with other medications: If you take oral contraceptives, certain diabetes medications, or other oral drugs, timing and absorption can affect how Mounjaro works. Discuss all medications with your prescriber.
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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.
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What To Do If Mounjaro Isn't Working
First, confirm you've been on your current Mounjaro dose for at least 4 weeks—the body needs time to adjust. If you're still in the titration phase, discuss when you can increase to the next dose with your prescriber. Review your diet and activity honestly: use our GLP-1 weight loss calculator to estimate realistic projections at your current dose with lifestyle changes factored in. Check whether you're experiencing appetite suppression—some people feel subtle effects rather than dramatic ones. Speak to your GP or the NHS tier 3 weight management service (if you access Mounjaro through NHS) about reviewing your progress. They may recommend a dose increase, switching to an alternative medication like Wegovy (semaglutide), or investigating underlying medical factors. Never stop or adjust your dose without professional guidance.
- 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.
How long should I wait before deciding Mounjaro isn't working?
Most prescribers recommend staying on your current Mounjaro dose for at least 4 weeks before assessing results, as the body needs time to adjust. You should also aim to reach at least a 10 mg dose (the therapeutic range) before concluding the medication isn't effective. Some people see results earlier; others need longer. Discuss your specific timeline with your prescriber.
Could my medications be interfering with Mounjaro?
Yes. Oral contraceptives, certain diabetes medications, and some other oral drugs can affect how well Mounjaro works or is absorbed. Always inform your prescriber of all medications you take. If you're on oral contraception, add a barrier method for 4 weeks after starting Mounjaro and after each dose increase, as absorption may be reduced.
What's the difference between Mounjaro and Wegovy if Mounjaro isn't working?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist; Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro showed slightly higher average weight loss in trials (~20.9% vs ~14.9%), but individual responses vary. If Mounjaro isn't working after proper titration and lifestyle changes, switching to Wegovy may help—this is a conversation for your prescriber.
Is 4 weeks on Mounjaro enough time to judge whether it's working?
No. Most guidelines suggest waiting at least 8–12 weeks at a therapeutic dose (10 mg or higher) before concluding the medication isn't effective. Mounjaro requires time for your body to adjust. Appetite suppression and weight loss can be gradual, especially in the first month.
Should I stop Mounjaro if it's not working?
Never stop your Mounjaro KwikPen without consulting your prescriber. If you're not seeing results, your GP or NHS tier 3 service may recommend a dose increase, a different medication, investigating underlying health factors, or intensifying lifestyle changes. Stopping suddenly can also cause rebound appetite. Always discuss next steps with your healthcare provider.
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