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 Does Your First Mounjaro Dose Take To Work?
The first dose of Mounjaro (tirzepatide) typically starts working within days, but full appetite-suppressing effects usually take 1–2 weeks to become noticeable. Most people remain on the starting dose of 2.5 mg for the first four weeks before their prescriber assesses progress and considers increasing to the next dose. It's important to understand that Mounjaro works gradually, and individual responses vary significantly. Results depend on your diet, activity level, and how your body responds to the medication. This guide explains the typical timeline and what to expect during your first weeks.
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.
Timeline: When Mounjaro Effects Typically Appear
Mounjaro begins working almost immediately after injection, as tirzepatide activates GIP and GLP-1 receptor pathways within hours. However, noticeable appetite reduction usually takes 3–7 days to develop. Most people report the strongest effects during week 1–2, though some feel changes within 24 hours.
- Days 1–3: Medication enters your system; mild effects may begin
- Days 4–7: Appetite suppression becomes more apparent for most people
- Week 2+: Full effects typically established; nausea may reduce as your body adjusts
- Week 4: Your prescriber reviews your response and considers dose escalation
The dose you start on (2.5 mg) is not a therapeutic dose—it's a tolerance and adjustment phase. Meaningful weight loss typically accelerates once you reach maintenance doses (5 mg and above) around weeks 5–8. Individual timelines vary based on body composition, metabolism, and lifestyle factors.
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 Affects How Quickly Mounjaro Works For You
Several factors influence how fast you'll notice Mounjaro's effects:
- Your starting weight and BMI: Higher body weight may take longer to show visible results
- Diet quality: Mounjaro suppresses appetite, but you must eat less to lose weight—it's not automatic
- Physical activity: Combined with exercise, weight loss accelerates significantly
- Metabolism: Genetics and age affect how quickly your body responds
- Other medications: Certain medications may interact or slow effects
- Adherence: Missing doses or incorrect injection technique reduces effectiveness
Track your progress using our free progress tracker to monitor changes over the first 4–8 weeks. Our GLP-1 weight loss calculator can also help you set realistic expectations for your dose level.
- 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.
Will I see weight loss in the first week of Mounjaro?
Most people do not see significant weight loss in the first week. Week 1 focuses on appetite suppression settling in. You may feel less hungry and eat smaller portions, which could result in 0.5–1 kg loss. Most noticeable weight loss begins after 2–3 weeks as the appetite reduction compounds with reduced calorie intake. Clinical trials showed meaningful weight loss within 4 weeks, but individual timelines vary.
What if Mounjaro doesn't work after the first dose?
The first dose (2.5 mg) is a starting dose, not a therapeutic dose. It's designed to help your body adjust and minimise side effects. Full effects emerge at higher doses. Always stay on each dose for the full 4 weeks before escalating. If you have concerns after 4 weeks, speak to your prescriber—they may adjust your approach or confirm the medication is working as expected for your body.
How does Mounjaro compare to Wegovy for speed of action?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) start working within days, but Mounjaro is a dual GIP and GLP-1 agonist whilst Wegovy activates only GLP-1. Clinical trials suggest Mounjaro may produce faster weight loss at equivalent timepoints, but both require 4+ weeks to show substantial results. Individual responses vary—neither is "faster" for everyone.
Should I change my diet immediately when starting Mounjaro?
Yes. Mounjaro works by reducing appetite, but weight loss requires a calorie deficit. Start eating smaller, nutrient-dense meals immediately. You may naturally eat less due to appetite suppression, but being intentional about diet choices accelerates results. Your prescriber may recommend dietary support or a structured weight loss programme alongside your injections.
When can I expect noticeable physical changes from Mounjaro?
Most people notice changes between weeks 2–4: looser clothing, different hunger patterns, and improved energy. Visible physical changes (face, body shape) typically appear after 8–12 weeks of consistent use at therapeutic doses. Use our BMI calculator to track your BMI progress alongside weight, and our progress tracker to log weekly measurements and photos for comparison.
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.