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 Long After a Mounjaro Injection Does It Start to Work?
Mounjaro (tirzepatide) is a prescription-only weight loss injection that doesn't work instantly — most people begin noticing appetite-suppressing effects within 3–7 days of their first injection, though the full effect builds gradually over several weeks. Understanding the timeline helps set realistic expectations when starting this dual GIP and GLP-1 receptor agonist. Your body's response depends on several factors, including dose level, individual metabolism, and how long you've been taking the medication. This guide explains what happens after each injection and when you can expect noticeable changes.
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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.
Timeline: When Mounjaro Effects Typically Appear
Days 1–3 after injection: Most people experience minimal appetite changes immediately. Some report mild nausea or stomach changes as their body adjusts to the medication.
Days 3–7: Appetite suppression usually becomes noticeable. You may feel fuller after smaller meals, experience reduced food cravings, and notice less interest in snacking between meals.
Weeks 2–4: Effects gradually strengthen, particularly if you're in your first month of treatment. Your body is adjusting to the weekly injection routine.
Weeks 4–12: By the time you reach your target maintenance dose, appetite control is typically well established. Weight loss becomes measurable on the scale, though results vary significantly between individuals.
- Each dose increase (from 2.5 mg up to 15 mg) may cause appetite effects to intensify
- Full therapeutic benefit at each dose level takes approximately 4 weeks to develop
- Individual variation is significant — some people feel effects sooner, others take longer
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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.
Dose Escalation and Effect Timeline
Mounjaro requires dose titration — you start at 2.5 mg weekly for 4 weeks, then increase by 2.5 mg every 4 weeks until reaching your optimal dose (typically 10–15 mg). This gradual escalation means the appetite-suppressing effects don't reach their peak until week 12 or beyond. Your GP or prescriber will manage this schedule based on your tolerance and response. Using our free GLP-1 weight loss calculator can help you understand typical weight loss projections at different dose levels. It's important to remain patient during dose escalation — rushing this process increases side effects without improving outcomes. Clinical trial data shows that meaningful weight loss becomes visible around weeks 8–12 for most people using a weekly weight loss injection, though some see changes earlier.
- 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 feel Mounjaro working immediately after my first injection?
No — most people don't feel appetite changes in the first 24–48 hours. Effects typically begin between days 3–7 as the medication accumulates in your system. Nausea is more common as an immediate response than appetite suppression.
How long before I see weight loss on the scale?
Weight loss usually becomes measurable after 2–4 weeks of consistent injections, though this varies. Clinical trials showed average weight loss accelerates from weeks 8–12 onwards. Patience during the early weeks is important.
Does Mounjaro work faster if I start at a higher dose?
No — starting at a higher dose increases side effects without accelerating results. The NHS-recommended titration schedule (starting 2.5 mg, increasing every 4 weeks) gives the best safety and efficacy balance.
What if I don't feel Mounjaro working after 4 weeks?
Some people notice subtle appetite changes others miss. Track eating patterns and fullness rather than just cravings. If genuinely no effect after 8 weeks at a stable dose, discuss with your prescriber — dose adjustment may be needed.
Can I use the MyWeightCheck BMI calculator to track if Mounjaro is working?
Yes — our free BMI calculator helps you monitor changes in your BMI alongside weight. This gives a fuller picture of progress, especially if weight loss is slow but body composition is improving.
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.