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
Does Mounjaro Work Instantly?
No, Mounjaro does not work instantly. Weight loss with this prescription injection typically takes several weeks to become noticeable, as your dose is gradually increased over time. In the UK, Mounjaro (tirzepatide) follows a structured dose titration schedule, meaning you start at a low dose and increase every 4 weeks. Most people begin to notice appetite changes and modest weight loss around weeks 4–8, though individual results vary significantly. Understanding the timeline helps set realistic expectations if you're considering NHS or private access to this medication.
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.
How Mounjaro's Dose Titration Works in the UK
Mounjaro requires a gradual dose increase schedule — it does not provide gradual improvement. You start at 2.5 mg once weekly, the lowest starting dose, which is designed to let your body adapt and minimise side effects. After 4 weeks, your prescriber increases the dose to 5 mg if tolerated well. The dose then escalates by 2.5 mg increments every 4 weeks (7.5 mg, 10 mg, 12.5 mg, 15 mg maximum), depending on your response and tolerability.
Most people reach a therapeutic maintenance dose between weeks 12–20. Weight loss during the initial weeks (0–4) is often minimal because the starting dose is relatively low. Noticeable appetite suppression and weight changes typically emerge once you reach the 5 mg or 7.5 mg dose. Because of this tirzepatide dose titration schedule, expecting gradual improvement is unrealistic — the process is designed for safety and tolerability, not rapid outcomes. Your GP or prescriber will monitor your progress at each dose level.
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.
NHS Access and Expected Timeline in the UK
If accessing Mounjaro through NHS specialist weight management services (tier 3), the timeline is often extended due to referral and assessment waiting times in many areas. You would first see your GP, receive a referral to your local integrated care board's specialist service, wait for an appointment, then begin the 4-week dose escalation cycle. In contrast, private prescribing through a UK-licensed online prescriber can start the medication faster, though the 4-week dose titration schedule remains the same.
Regardless of route, clinical trials (SURMOUNT-1) showed average weight loss of approximately 15–21% of body weight by week 72 when combined with reduced-calorie diet and physical activity. Try our GLP-1 weight loss calculator to see projected weight loss at different Mounjaro doses, and use the BMI calculator to track your progress alongside the medication. Individual responses vary — some people see results sooner, others take longer.
- 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 start working?
Mounjaro typically begins to show effects within 2–4 weeks at the starting dose of 2.5 mg, though noticeable appetite suppression and weight loss usually emerge at 5 mg or higher. Full therapeutic benefit takes 12–20 weeks as you reach your maintenance dose. Results vary between individuals.
Will I lose weight in the first week on Mounjaro?
No, significant weight loss in the first week is unlikely. The starting 2.5 mg dose is low by design to minimise side effects. Most weight loss occurs once your dose reaches 5–7.5 mg, typically by weeks 5–8. Consistent results require adherence to the medication schedule plus dietary and lifestyle changes.
Can Mounjaro be prescribed faster through the NHS?
NHS access typically requires a GP referral to tier 3 specialist weight management services, which can involve waiting lists of several weeks or months depending on your area. The 4-week dose titration schedule itself cannot be accelerated — it's a safety requirement. Private prescribing may be faster to initiate but follows the same dose schedule.
What's the difference between Mounjaro and Wegovy regarding speed of action?
Both Mounjaro and Wegovy require gradual dose titration and neither works instantly. Mounjaro uses a dual GIP and GLP-1 mechanism and reaches maximum dose (15 mg) faster than Wegovy (2.4 mg), but the timeline to noticeable effects is similar—usually weeks 4–8. Individual response varies with both medications.
Should I expect instant appetite suppression from Mounjaro?
Appetite suppression is not instant. At the 2.5 mg starting dose, you may notice minimal change. Most people report meaningful appetite reduction once they reach 5 mg or higher, typically by week 5 onwards. This gradual approach helps your body adapt and reduces side effects. Your GP or prescriber can advise on your individual response.
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.