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 Peak and How Long Does It Take?
Mounjaro (tirzepatide) doesn't reach peak effectiveness immediately — it takes several weeks for the medication to build up in your system and for your body to respond. Most people see the most noticeable effects after 8–12 weeks of treatment, though individual timing varies significantly. Understanding when to expect changes helps you set realistic expectations and stay motivated during the early stages of your prescription weight management journey.
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 Escalation Affects Peak Timing
Mounjaro works through a gradual dose escalation schedule, which directly influences when peak effects occur. You start at 2.5 mg for the first four weeks, then increase by 2.5 mg increments every four weeks (5 mg, 7.5 mg, 10 mg, 12.5 mg, and finally 15 mg as the maximum dose). Each dose increase takes approximately four weeks to be fully absorbed and processed by your body. This means your personal peak effectiveness depends on which dose you stabilise on — someone staying at 5 mg will reach their peak sooner than someone escalating to 15 mg. Clinical trial data shows that weight loss continues to increase progressively with higher doses, so reaching your optimal dose (the one your prescriber determines is right for you) is when you'll experience your maximum effect. Most people notice appetite reduction within the first 1–2 weeks, but significant weight loss typically appears after 4–8 weeks of consistent dosing.
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.
What to Expect During the First 12 Weeks
In weeks 1–4, you may experience initial side effects like nausea or appetite changes, but dramatic weight loss is unlikely — this is the adjustment phase. By weeks 5–8 (when most people reach 5 mg), appetite suppression becomes more noticeable and modest weight loss may appear on the scales or progress tracker. Weeks 9–12 typically bring more visible results as your body adapts and, if you're escalating doses, you may be moving toward your maintenance dose. Individual variation is substantial — some people see rapid changes whilst others progress more gradually. Your prescriber or GP can help monitor your progress and adjust your dose if needed. Remember that Mounjaro works best alongside a reduced-calorie diet and increased physical activity; medication alone isn't a substitute for lifestyle changes. Track your weight loss journey using our free progress tracker to see patterns over time.
- 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 see weight loss in the first week of Mounjaro?
Unlikely. Most people don't see measurable weight loss in week one. Initial changes are usually appetite reduction and possible side effects like nausea. Significant weight loss typically appears after 4–8 weeks as your body adjusts to the medication and the dose builds up in your system.
At what dose does Mounjaro become most effective?
Clinical trials show progressive weight loss improvement with higher doses. The 15 mg dose (maximum) produced the greatest average weight loss (~21% body weight), but your prescriber will determine your optimal dose based on your individual health and response. Some people achieve good results at 10 mg.
How long does it take to reach the maximum Mounjaro dose?
Reaching 15 mg takes a minimum of 20 weeks (five four-week escalation periods). However, your prescriber may keep you at a lower maintenance dose if that's working well for you — there's no requirement to reach the maximum dose.
Can Mounjaro peak and then stop working?
Mounjaro's effect doesn't typically plateau and stop. Some people experience a slight plateau in weight loss after several months, which is normal with any weight loss treatment. Continuing healthy eating and physical activity helps maintain progress. Discuss any concerns with your GP.
What's the difference between Mounjaro peak timing and Wegovy peak timing?
Wegovy (semaglutide) also requires 4–8 weeks to show significant effects, but uses a different dose schedule (starting 0.25 mg, maximum 2.4 mg). Mounjaro's dual GIP/GLP-1 mechanism means dose titration differs. Both reach their individual peaks based on their maximum doses — around 8–12 weeks on average for noticeable results.
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.