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 Mounjaro Injection Does It Work?
Mounjaro (tirzepatide) typically begins to work within the first few days of your initial injection, though the effects build gradually over several weeks. Most people notice a measurable difference in appetite and how quickly they feel full by weeks two to three. Results continue to improve as your dose increases through the titration schedule. Understanding how Mounjaro works over time, plus the UK prescription options available, can help you plan your weight management journey effectively.
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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.
When Mounjaro Starts Working — Timeline and Expectations
Mounjaro begins affecting appetite regulation and satiety within 3 to 5 days of your first injection, as tirzepatide activates GIP and GLP-1 receptors in the brain. However, noticeable weight loss typically appears over 4 to 8 weeks as the cumulative effect builds. By 12 to 16 weeks (three to four months), most people experience more significant appetite suppression and weight reduction.
The timeline varies between individuals — some notice results sooner, others take longer. During the dose titration phase (weeks 1–20), your dose increases gradually, and effects tend to strengthen at each step. Most clinical data shows meaningful weight loss emerges by the 12-week mark, with continued improvement through week 72 in trials.
- Weeks 1–2: Mild appetite reduction may begin
- Weeks 3–8: Noticeable change in hunger and portion sizes
- Weeks 9–16: Significant appetite suppression and early weight loss
- Weeks 17+: Maximum effect at maintenance dose
Patience is essential — Mounjaro is not a rapid-action injection. It works best alongside a reduced-calorie diet and increased physical activity. Try our GLP-1 weight loss calculator above to project potential outcomes at your expected dose.
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.
UK Prescription Access and Cost Options for Mounjaro
In the UK, Mounjaro is available through two main routes: NHS specialist services and private prescription. NHS access requires referral to a tier 3 or tier 4 specialist weight management service via your GP, though waiting lists can be lengthy in many regions. NICE has approved tirzepatide for NHS use, but availability and speed of access depend on your local Integrated Care Board.
Private prescription provides faster access without waiting lists. A UK-licensed prescriber conducts an online clinical assessment, and if suitable, issues a prescription for a GPhC-registered pharmacy to dispense. Private costs typically range from £129 to £320 monthly depending on your dose — the 2.5 mg starting dose costs less than the 15 mg maintenance dose. Both routes require an initial assessment to confirm eligibility and suitability.
- 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 does Mounjaro suppress appetite?
Initial appetite reduction begins within 3–5 days of injection, but noticeable effects are typically clearer by week 2–3. Maximum appetite suppression develops gradually as your dose increases through titration. Individual timelines vary considerably.
Can I speed up how Mounjaro works?
No — Mounjaro has a fixed dose escalation schedule (minimum 4 weeks per dose) for safety. Faster increases are not recommended. Results are best with dietary changes and physical activity alongside the injection.
What is the difference between NHS and private Mounjaro access in the UK?
NHS access is via specialist referral with no direct cost but significant waiting lists in many areas. Private prescription is faster, requires an online clinical assessment, costs £129–£320 monthly, and does not require NHS referral.
Why does Mounjaro take weeks to show weight loss if it starts working in days?
Appetite suppression begins early, but measurable weight loss requires sustained calorie deficit over time. Clinical trials show meaningful results by 12 weeks. Weight loss is cumulative and builds as dose increases.
How much weight will I lose, and how quickly?
Clinical trials show average weight loss of 15–21% of body weight over 72 weeks, but individual results vary significantly. Safe loss is typically 0.5–1 kg per week. Use our GLP-1 weight loss calculator to see projected ranges for your situation.
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.