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 Not Work for Weight Loss?
Yes, Mounjaro (tirzepatide) does not work for everyone, and some people experience minimal weight loss or weight loss plateaus during treatment. While clinical trials showed average weight loss of 15–21% of body weight, individual results vary significantly. Some people may see little to no change, whilst others respond excellently. Understanding why treatment may not deliver expected results—and what options exist if this happens—is important for anyone considering or already using this prescription medication.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Not Deliver Expected Results
Several factors can explain why Mounjaro may not work as expected. Insufficient dose is common—many people are still in the dose titration phase and haven't reached their optimal dose (maximum 15 mg once weekly). The standard escalation takes 16–20 weeks to reach the highest dose, so early plateau may simply reflect an inadequate dose for your body. Lifestyle factors matter significantly; Mounjaro works alongside reduced-calorie diet and physical activity, not instead of them. If dietary habits haven't changed or calorie intake remains high, weight loss will plateau. Individual biology varies—some people are naturally less responsive to GLP-1 and GIP receptor agonists due to genetics, metabolism, or existing health conditions. Medication interactions can reduce effectiveness (e.g., oral contraceptives may reduce absorption of other medications). Additionally, after 6–12 months of use, some people experience a weight loss plateau where the body adapts—this is normal and does not mean the medication has "stopped working."
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.
What to Do If Mounjaro Isn't Working
If you're not seeing expected weight loss, discuss this openly with your prescriber or GP. Do not increase your dose without guidance—your prescriber will assess whether you've reached your optimal dose and whether escalation is appropriate. Request a review of your diet and physical activity; a structured approach with support from a dietitian can unlock results. Some people respond better to Mounjaro vs Wegovy (semaglutide uses a single GLP-1 mechanism, whilst Mounjaro uses a dual GIP and GLP-1 mechanism)—if you're on one, switching to the other may help, though this requires prescriber assessment. Weight loss is not assured, and if Mounjaro genuinely isn't working after optimisation, your GP can discuss alternatives including other weight loss medications, lifestyle intervention programmes, or referral to NHS specialist weight management services.
- 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 before I know if Mounjaro is working?
Weight loss typically begins within 2–4 weeks, but meaningful results usually appear after 8–12 weeks. However, the dose titration phase takes 16–20 weeks to complete. If you're still escalating doses, you haven't reached your optimal therapeutic dose yet. Don't judge effectiveness until you've been on a stable dose for at least 4–8 weeks.
Does Mounjaro stop working over time?
Some people experience a weight loss plateau after several months—this is the body adapting, not the medication "stopping." This is normal and doesn't mean treatment has failed. Your prescriber can review your dose, diet, and activity level to determine next steps.
Can genetics affect how Mounjaro works?
Yes. Genetic variation affects how individual bodies respond to GLP-1 and GIP receptor agonists. Some people are naturally less responsive due to their genetic makeup, metabolism, or underlying health conditions. This doesn't reflect medication quality—it reflects individual biology.
What if I've reached 15 mg and still not losing weight?
If you're on the maximum dose (15 mg once weekly) and seeing minimal results after 8+ weeks of stable dosing, discuss with your prescriber. Options include reviewing diet and activity, switching to an alternative medication like Wegovy, or exploring whether Mounjaro is genuinely the right choice for your body.
Can diet and exercise affect how well Mounjaro works?
Absolutely. Mounjaro works best alongside a reduced-calorie diet and regular physical activity. If calorie intake remains high or activity levels are very low, weight loss will be minimal regardless of medication. Using our GLP-1 weight loss calculator can help you understand projected results with realistic diet and activity changes.
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.