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
When Does Mounjaro Start Working in the UK?
Mounjaro (tirzepatide) typically begins showing weight loss effects within 2–4 weeks of starting treatment, though individual responses vary significantly. Most people see more noticeable results after 8–12 weeks as their body adjusts to the medication and doses are gradually increased. The timeline also depends on your starting dose, how quickly your dose is escalated, and your personal metabolism. Understanding when to expect results helps you set realistic goals alongside lifestyle changes.
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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: How Quickly Does Mounjaro Work?
Weeks 1–4 (Starting Phase): You'll begin on a 2.5 mg dose. Many people experience reduced appetite and early appetite suppression within the first week, though weight loss may not yet be visible on the scales. Nausea is common during this phase and usually improves after a few days.
Weeks 5–8 (Dose Increase to 5 mg): After a minimum of 4 weeks, your dose increases to 5 mg. Clinical trial data suggests weight loss acceleration typically begins around week 6–8 as the medication reaches steady state and appetite suppression deepens. Most people report feeling fuller faster and having fewer cravings.
Weeks 9–16+ (Escalation Phase): Doses continue escalating every 4 weeks (7.5 mg, then 10 mg, 12.5 mg, up to 15 mg maximum). Significant weight loss usually becomes apparent by weeks 12–16 for most users. The longer you remain on a stable dose, the more consistent results become. Our GLP-1 weight loss calculator can help you estimate potential outcomes at different dose levels based on clinical trial data.
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 Context
Mounjaro is a prescription-only medicine in the UK and requires clinical assessment before dispensing. Access falls into two main routes: NHS specialist weight management services (tier 3) or private online prescribing through GPhC-registered pharmacies.
NHS Route: Requires referral from your GP to a specialist weight management service. NICE has approved tirzepatide for use within NHS services, but regional availability and waiting times vary considerably — some areas have 6–12 month waiting lists.
Private Route: Online consultation with a UK-licensed prescriber, typically costing £129–£320 per month depending on your dose. This route bypasses NHS waiting lists but requires upfront payment. A prescriber will assess your eligibility (BMI ≥27.5 with health conditions, or ≥30 without) during an online consultation before issuing a prescription.
- 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 or does it take weeks?
Mounjaro does not provide gradual improvement. Appetite suppression may begin within days, but meaningful weight loss typically appears after 4–8 weeks of consistent use. Results accelerate as your dose increases during the titration phase. Individual timelines vary based on metabolism, diet, and exercise.
What is the Mounjaro KwikPen and how does the dose titration work?
The Mounjaro KwikPen is a pre-filled injection device delivering one weekly dose. Dose titration means your prescriber increases your dose every 4 weeks: starting at 2.5 mg, then 5 mg, 7.5 mg, 10 mg, 12.5 mg, and up to 15 mg maximum. Each dose step is held for a minimum of 4 weeks.
How much weight loss can I expect with Mounjaro in the first 12 weeks?
In clinical trials, average weight loss after 12 weeks was modest — typically 2–5 kg — because users are still in the dose titration phase. Significant weight loss accelerates between weeks 12–20 as doses reach therapeutic levels. By 72 weeks at maximum dose, average weight loss was 15–21% of body weight, though individual results vary considerably.
Is Mounjaro available on the NHS in the UK and how long are waiting times?
Yes, NICE approved tirzepatide (Mounjaro) for NHS weight management services. However, access is through tier 3 specialist services and availability varies by region. Many areas have 6–12 month waiting lists. Your GP can refer you, but eligibility depends on your BMI, health conditions, and local service availability.
What's the cost difference between NHS and private Mounjaro prescriptions in the UK?
NHS access is free but involves waiting lists. Private prescriptions cost approximately £129–£320 per month depending on dose, with lower doses (2.5–5 mg) at the cheaper end and maximum doses (15 mg) at the higher end. Private online prescribing provides faster access without NHS waiting times but requires upfront payment.
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.