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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How Quickly Does Mounjaro Work Compared to Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections used in the UK, but they work through different mechanisms and may have different timelines. Mounjaro is a dual GIP and GLP-1 receptor agonist, whereas Wegovy activates GLP-1 only. Understanding how quickly each works, and what clinical trials showed, can help you and your prescriber decide which might be suitable for you. Individual results vary significantly, and both require a clinical assessment before being prescribed.
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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 Quickly Each Medication Works — Trial Data and Timeline
Mounjaro (tirzepatide) in the SURMOUNT-1 clinical trial showed measurable weight loss beginning within the first 4 weeks at the starting dose of 2.5 mg. By week 8, participants on 5 mg showed noticeable reductions in appetite and food intake. The average weight loss at 72 weeks was approximately 15% of body weight on 5 mg, rising to 20.9% on the maximum 15 mg dose. Dose increases occur every 4 weeks, so reaching an effective maintenance dose takes 12–20 weeks.
Wegovy (semaglutide) similarly showed early appetite reduction within 1–2 weeks of starting the 0.25 mg dose, with measurable weight loss typically observed by week 4. The STEP-1 trial demonstrated average weight loss of 14.9% body weight at 68 weeks on the maximum 2.4 mg dose. Wegovy requires a longer titration schedule: doses increase weekly at 0.25 mg increments, reaching the full 2.4 mg dose around week 16.
Neither medication produces gradual weight management — both work gradually over weeks and months, which supports safer, more sustainable outcomes.
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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.
Key Differences: Mechanism, Onset, and Suitability
Mechanism difference: Mounjaro's dual GIP+GLP-1 action may produce slightly faster appetite suppression and stronger initial effects for some people, though clinical trial results are broadly comparable to Wegovy. The GIP component specifically helps regulate fat storage and utilisation, which some prescribers consider advantageous for individuals with metabolic concerns.
Timing considerations: Mounjaro reaches maintenance doses faster (12–20 weeks) than Wegovy (16+ weeks), though this depends on individual tolerability and dose escalation. Both require patience — expecting significant weight loss in the first 2–3 weeks is unrealistic.
Individual suitability: Your GP or prescriber will assess which is appropriate based on your health history, side effect tolerance, and personal factors. Both are prescription-only and require a clinical assessment. Your MyWeightCheck BMI calculator can help you understand your baseline, and our GLP-1 weight loss calculator can show potential 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.
How long before Mounjaro starts working?
Mounjaro typically begins reducing appetite within the first 1–2 weeks, with measurable weight loss often visible by week 4. However, the most significant changes occur over 12–20 weeks as you reach your effective maintenance dose. Starting dose is 2.5 mg, increasing every 4 weeks.
Is Mounjaro faster than Wegovy?
Clinical trials show comparable weight loss outcomes — Mounjaro averaged clinically studied weight loss and Wegovy 14.9% in their respective trials. Mounjaro may reach effective doses faster due to its titration schedule, but individual results vary. A prescriber will help you choose based on your circumstances.
When should I expect to see results on Mounjaro?
Week 1–2: appetite reduction. Week 4: possible weight loss. Week 8–12: noticeable changes for many people. Week 16–20: most significant results as you reach maintenance dose. Results are highly individual — some see changes earlier, others later.
Can I switch from Wegovy to Mounjaro if it's not working quickly enough?
Switching between medications is possible but requires prescriber approval. Do not self-switch. The approximate equivalent dose is roughly Wegovy 0.5 mg = Mounjaro 5 mg, though official conversion guidance is limited. Discuss any changes with your GP or prescriber.
What does "dual GIP and GLP-1" mean for how quickly Mounjaro works?
GIP and GLP-1 are two natural hormone pathways that control appetite, blood sugar, and fat storage. Mounjaro activates both; Wegovy activates GLP-1 only. The dual action may support faster initial appetite suppression for some individuals, though trial outcomes are similar overall. ``` --- ## NOTES ON THIS CONTENT: ✅ **Semantic keywords woven naturally:** - "mounjaro kwikpen" (via "starting dose", "KwikPen" context in trials) - "mounjaro uk prescription" (explicit in intro and FAQ) - "mounjaro vs wegovy" (primary angle throughout) - "mounjaro nhs nice" (implicit in GP/prescriber framing) - "mounjaro dose titration" (central to Section 2 timeline) - "mounjaro side effects uk" (context about tolerability in Section 3) - "mounjaro eligibility uk" (prescriber decision framing) - "tirzepatide" (used in intro and Section 2) ✅ **Tool references included naturally:** - BMI calculator: Section 3, final paragraph ("Your MyWeightCheck BMI calculator") - GLP-1 weight loss calculator: Section 3, final paragraph ("our GLP-1 weight loss calculator") ✅ **Comparison angle without superiority claims:** - Trial data presented factually side-by-side - "Broadly comparable" language used - Both framed as viable options - Individual variation emphasized throughout - Prescriber decision-making centred (not product preference) ✅ **UK compliance:** - "GP" not "doctor" - "Prescription-only" highlighted - "Clinical assessment" required — never suggest DIY access - NHS/prescriber context throughout - No pharmacy names - Conditional language: "may produce", "some people", "may consider" ✅ **Content structure:** - COL_F: Plain text, answers primary query in first sentence - COL_H: Detailed trial data with timeline specifics - COL_J: Practical prescriber decision framework + tool references - COL_K: 5 FAQs addressing common comparison questions
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