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 Is Mounjaro At Its Peak?
Mounjaro (tirzepatide) typically reaches its peak effectiveness around week 20–24 after starting treatment, when most patients have reached the maximum maintenance dose of 15 mg. However, "peak" doesn't mean sudden — weight loss continues gradually throughout the dose escalation phase. The timing and magnitude of results depend on individual factors including starting weight, metabolism, adherence to diet and exercise, and how your body responds to the medication. Your prescriber will monitor your progress and adjust your dose based on your personal response. Individual results vary significantly, and some people reach satisfactory results at lower doses before reaching the maximum.
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 Dose Titration Affects Peak Timing
Mounjaro uses a structured dose titration schedule — you don't start at the full dose. You begin at 2.5 mg weekly, then increase by 2.5 mg every 4 weeks: 5 mg (week 5), 7.5 mg (week 9), 10 mg (week 13), 12.5 mg (week 17), and finally 15 mg (week 21). Each dose escalation is a separate injection via the Mounjaro KwikPen. Most people continue increasing until they reach 15 mg or find a dose that delivers satisfactory results. Clinical trial data from SURMOUNT-1 showed average weight loss of approximately 20.9% body weight at the 15 mg dose over 72 weeks, though results were substantial even at lower doses — 10 mg produced around 19.5% average weight loss. Peak effectiveness is when you've been stable on your maintenance dose for several months, allowing the full metabolic effects to stabilise.
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.
Mounjaro vs Wegovy — Peak Timing and Mechanism Differences
Wegovy (semaglutide) is a GLP-1 only agonist, whilst Mounjaro is a dual GIP/GLP-1 receptor agonist — this mechanistic difference affects how and when peak effects occur. Wegovy reaches its maximum dose of 2.4 mg by week 17 (five escalation steps from 0.25 mg). STEP trial data showed approximately 14.9% average weight loss at the 2.4 mg dose. Mounjaro's dual mechanism may contribute to its higher average weight loss in SURMOUNT trials, but individual responses vary considerably. Both medications reach peak therapeutic effect once stable on maintenance dose for 4–8 weeks. Neither is assured — prescriber suitability assessment determines which is appropriate for you based on your medical history, other medications, and individual health profile.
- 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 better at 15 mg than at 10 mg?
Clinical trials showed higher average weight loss at 15 mg (~20.9%) versus 10 mg (~19.5%), but individual variation is significant. Some people achieve good results at lower doses and experience more side effects at maximum dose. Your prescriber will assess the balance of benefit and tolerability for your situation.
How long does it take to feel Mounjaro working?
Most people notice reduced appetite and fuller feeling within 1–2 weeks of starting, but weight loss acceleration typically increases as you escalate doses over the first 12–16 weeks. Peak metabolic effects usually stabilise by week 20–24 once you've reached and maintained your target dose.
Is Mounjaro peak the same for everyone?
No. "Peak" varies by individual factors: starting weight, metabolism, adherence to diet and exercise, other medications, and how your body responds to tirzepatide. Some people peak at 10 mg, others at 15 mg. Your GP or prescriber monitors your progress and optimises your personal peak dose.
When should I switch to Wegovy if Mounjaro isn't working?
This is a prescriber decision based on your individual response and tolerance. Don't stop or switch medications without clinical guidance. If you're not seeing expected results after 3–4 months on a stable dose, discuss this with your prescriber — they may adjust your dose or explore alternatives.
Can I use the MyWeightCheck GLP-1 calculator to estimate my peak weight loss?
Yes. Our free GLP-1 weight loss calculator can help you estimate potential weight loss projections at different Mounjaro and Wegovy doses, alongside clinical trial averages. Remember individual results vary — use it as a guide, not a assures. Combine it with our BMI calculator to track your starting point and healthy weight range. ```
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.