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 Before Mounjaro Starts to Work?
Most people notice their first appetite-reducing effects from Mounjaro within 1–2 weeks of starting treatment, though meaningful weight loss typically becomes noticeable after 4–8 weeks. However, everyone responds differently depending on dose, starting weight, and individual metabolism. This guide explains the expected timeline for Mounjaro in the UK, how the dose titration process works, and what NHS guidance says about realistic expectations.
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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.
Timeline: When Mounjaro Effects Appear
Week 1 (Starting dose 2.5 mg): Some users report mild appetite reduction and possible nausea. Stomach side effects (nausea, constipation, diarrhoea) are most common in the first week and often improve as your body adjusts.
Weeks 2–4: Appetite suppression typically becomes noticeable. You may naturally eat smaller portions and feel satisfied sooner. Early weight changes are usually modest.
Weeks 5–8 (Dose increased to 5 mg): Weight loss momentum typically increases. Clinical trials showed participants lost approximately 5–7% of their starting weight by week 8, though this varies.
Weeks 9–16 (Further dose increases): As your dose escalates toward the maintenance level, weight loss continues to accelerate. The tirzepatide dual GIP and GLP-1 mechanism becomes more fully active at higher doses.
Weeks 17+ (Maintenance dose): Peak weight loss effects are typically observed at your target maintenance dose. The SURMOUNT-1 trial showed average losses of 15–21% body weight at doses of 10–15 mg over approximately 72 weeks.
- Full effects are not usually realised until 12–16 weeks of consistent treatment
- Individual timelines vary significantly based on metabolism, diet adherence, and activity level
- Weight loss is not linear — expect week-to-week variation
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.
NHS Mounjaro, NICE Guidance, and Realistic Expectations
The NHS and NICE have approved tirzepatide for weight management in specialist tier 3 services. However, NHS access varies significantly by region — some areas have year-long waiting lists. NICE guidance emphasises that weight loss medications should be combined with reduced-calorie diet and increased physical activity for optimal results. Without lifestyle changes, Mounjaro alone produces far less weight loss.
On the NHS, your GP cannot prescribe Mounjaro directly — you must be referred to a specialist weight management service, which involves assessment and ongoing monitoring. Private routes via UK-licensed prescribers typically offer faster access. Whichever route you take, your prescriber will monitor side effects and adjust your mounjaro kwikpen dose based on your response and tolerance.
Use our free GLP-1 weight loss calculator above to estimate your potential weight loss at different doses, and try our BMI calculator to check where you stand today against NHS healthy weight ranges.
- 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 quickly will I see weight loss on Mounjaro?
Modest weight changes may appear by week 4–6, but significant weight loss typically becomes noticeable between weeks 8–12. The SURMOUNT trial showed average weight loss plateaus at around 72 weeks. Remember that individual results vary — diet, activity level, and metabolism all affect how quickly you lose weight.
Why does Mounjaro take several weeks to work?
You start on a low dose (2.5 mg) to allow your body to adjust and minimise side effects. Your dose increases every 4 weeks toward the maintenance level. Peak weight loss effects aren't achieved until you reach your target dose, which typically takes 16–20 weeks. This gradual titration is intentional and safer.
Can I get Mounjaro through the NHS in the UK?
Yes, NICE has approved tirzepatide for NHS specialist weight management services. However, access is through tier 3 referral only — your GP cannot prescribe it directly. Waiting times vary by region and can be several months to over a year in some areas. Private access via UK-licensed prescribers is typically faster.
What if Mounjaro isn't working after 8 weeks?
Give it longer — most people don't see peak effects until week 12 or beyond, when they've reached a higher dose. If you're experiencing severe side effects early on, your prescriber may slow your dose escalation. If you're not losing any weight by 12 weeks despite diet changes, discuss this with your prescriber — they may adjust your approach.
Is Mounjaro the same as Wegovy?
No. Both are prescription-only weight loss injections in the UK, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials suggest Mounjaro may produce slightly more weight loss on average, but individual responses vary. Your prescriber will assess which suits you best.
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.