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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Will Wegovy Work If Mounjaro Didn't?
If Mounjaro (tirzepatide) hasn't delivered the weight loss results you hoped for, you might be wondering whether Wegovy (semaglutide) could work better. The answer isn't straightforward — response to GLP-1 medications varies significantly between individuals, and switching from one to another may help, but isn't assured. Both are prescription-only weight loss injections licensed in the UK, but they work slightly differently. Understanding the distinction and having an honest conversation with your prescriber about why Mounjaro didn't work for you is essential before considering a switch.
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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 Differ in Mechanism
The key difference is in how these medications work at a hormonal level. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two separate hormone pathways to regulate appetite and blood sugar control. Wegovy (semaglutide) is a GLP-1 agonist only — it activates just the GLP-1 pathway. This means Mounjaro has two mechanisms working together, whilst Wegovy relies on a single mechanism. In clinical trials, this dual action helped participants lose an average of approximately 15–21% of body weight with Mounjaro, compared to roughly 15% with Wegovy (STEP trial data) — though individual results vary considerably. For some people, the dual mechanism of Mounjaro works better; for others, a single mechanism like Wegovy may be more effective or cause fewer side effects. Your body's response depends on your unique physiology, genetics, and how your GIP and GLP-1 pathways function.
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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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Why Mounjaro May Not Have Worked for You
If Mounjaro didn't deliver expected results, several factors could explain this. You may not have reached an effective dose — Mounjaro requires dose titration over weeks, starting at 2.5 mg and escalating to a maximum of 15 mg, and some people respond better at higher doses than they initially tried. Side effects like nausea or constipation may have caused you to stay on a lower dose longer than ideal. Alternatively, your body may simply not respond as well to the tirzepatide mechanism — this is entirely normal and doesn't predict how you'll respond to semaglutide. Adherence also matters: weight loss medications work best alongside a reduced-calorie diet and increased physical activity. Finally, your prescriber may not have assessed lifestyle factors thoroughly. Before switching, discuss with your GP or prescriber what specifically didn't work, whether you reached an adequate dose, and whether adjusting diet or exercise might have made a difference.
- 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.
Is Wegovy assured to work if Mounjaro didn't?
No. Wegovy (semaglutide) works differently from Mounjaro (tirzepatide) — it activates only the GLP-1 pathway rather than both GIP and GLP-1. Some people respond better to semaglutide; others don't. Individual response varies based on genetics, metabolism, and how your body responds to each mechanism. Your prescriber will assess whether switching makes sense for you.
Can I switch from Mounjaro to Wegovy on the NHS?
Access depends on your regional NHS authority. Some specialist weight management services may offer a switch, though NICE guidance typically considers tirzepatide first-line for weight management. Waiting lists and availability vary significantly across the UK. Your GP can refer you to a tier 3 specialist service to discuss options.
What's a rough dose equivalent between Mounjaro and Wegovy?
Approximate equivalence: Mounjaro 5 mg ≈ Wegovy 0.5 mg; Mounjaro 10 mg ≈ Wegovy 1.7 mg; Mounjaro 15 mg ≈ Wegovy 2.4 mg. These are approximate only. Any switch must be managed by your prescriber — there is no official conversion, and your prescriber will determine the right starting dose for you.
Will switching help if I had side effects on Mounjaro?
Possibly. Mounjaro and Wegovy cause similar side effects (nausea, constipation, diarrhoea, stomach pain), but severity and frequency vary between people. Some individuals tolerate semaglutide better than tirzepatide. If Mounjaro side effects were problematic, discuss this with your prescriber before switching — they may adjust dose, timing, or recommend an alternative approach rather than switching to another injection.
How long should I try Mounjaro before considering a switch?
Most prescribers recommend at least 12–16 weeks (3–4 months) at an effective dose to assess whether Mounjaro is working for you. This allows time for dose escalation and your body to adapt. If you're not seeing any weight change after reaching a stable dose and maintaining dietary changes, switching may be worth discussing with your prescriber. Our GLP-1 weight loss calculator can help you estimate realistic outcomes.
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