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
How Long After Injection Does Mounjaro Work?
Mounjaro (tirzepatide) begins working within hours of your first injection, but noticeable appetite reduction typically occurs within 2–4 days. However, meaningful weight loss usually takes 4–6 weeks to become apparent. This is because Mounjaro activates GLP-1 and GIP receptors in your brain and gut, signalling fullness and reducing food cravings. Individual timelines vary—some people feel effects sooner, while others may need several weeks on the same dose before noticing changes. Weight loss as a weekly weight loss injection depends on consistent use alongside dietary changes and increased physical activity, in line with NHS guidance.
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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.
How Mounjaro Works and Initial Timeline
Mounjaro starts acting immediately after injection, but the timeline for noticeable effects varies between individuals. Here's what typically happens:
- First 24 hours: Tirzepatide circulates in your bloodstream and begins binding to GLP-1 and GIP receptors. You may not notice anything yet.
- Days 2–4: Most people report reduced appetite, less interest in food, or feeling fuller after smaller meals. Some experience mild nausea (common and usually temporary).
- Week 2–4: Appetite suppression deepens as the dose stabilises in your system. Energy levels and food cravings typically reduce further.
- Week 4–6: Weight loss becomes measurable on scales. This is when most users see the first 1–2 kg lost, though results vary significantly based on diet and activity level.
Remember: Mounjaro's effectiveness is maximised when combined with a reduced-calorie diet and physical activity. Clinical trials showed dose titration—gradually increasing from 2.5 mg to 15 mg—produces better sustained results than staying on lower doses.
Clinically BackedWeight Loss
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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.
Eligibility, Access, and What Happens Next
In the UK, Mounjaro is a prescription-only medicine available through NHS specialist weight management services or private online prescribing following clinical assessment. You'll be eligible if your BMI is 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, hypertension, or sleep apnoea. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower thresholds apply.
Once prescribed, you'll receive a KwikPen with four weekly doses. Your prescriber will advise when to expect results—typically visible weight loss takes 4–6 weeks, with continued loss over 12–16 weeks as your dose increases. Our free GLP-1 weight loss calculator can help you understand realistic timelines based on your starting weight and health profile. If you don't notice appetite changes within 2–3 weeks on your starting dose, contact your prescriber.
- 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 work immediately after the first injection?
Mounjaro begins working within hours as tirzepatide enters your bloodstream, but noticeable appetite reduction typically takes 2–4 days. Meaningful weight loss usually appears after 4–6 weeks of consistent weekly injections. Everyone responds differently—some feel effects sooner, others take longer to notice changes, particularly if metabolism or activity levels vary.
What if I don't feel Mounjaro working after a week?
It's normal not to feel significant effects within one week. Most people notice appetite changes by day 4–7, but weight loss can take 4–6 weeks to become apparent on scales. If you've been on your starting dose (2.5 mg) for 4 weeks with no appetite change, contact your prescriber—dose titration may be adjusted.
How does Mounjaro compare to Wegovy for speed of action?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly weight loss injection options in the UK. Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy works through GLP-1 only. Clinical trials showed similar timelines: appetite suppression within days, measurable weight loss within 4–6 weeks. Individual response varies; neither is universally faster than the other.
Can I expect steady weight loss every week after Mounjaro starts working?
Weight loss with Mounjaro is not linear—you may lose 1–2 kg in week four, then plateau for a week or two before losing more. This is normal and expected. Sustained weight loss depends on maintaining your reduced-calorie diet and physical activity alongside the injection. Your prescriber can advise on realistic targets based on your health profile.
What should I do if Mounjaro isn't working for me after 8 weeks?
If you're on a stable dose (typically 5 mg or higher) for 4+ weeks and seeing no appetite suppression or weight loss, contact your prescriber. They may: adjust your dose further, review your diet and activity level, check for underlying conditions affecting metabolism, or discuss alternative options. Mounjaro works best as part of a comprehensive lifestyle approach aligned with NHS weight management principles.
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.