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
What Happens Each Week on Mounjaro — UK Timeline
Starting Mounjaro involves a gradual dose escalation over several months, not an immediate jump to the maximum dose. This week-by-week timeline explains what to expect during your first weeks on tirzepatide, how the dosing schedule works in the UK, and when you might notice changes. Understanding the timeline helps you prepare for side effects and set realistic expectations for weight management results.
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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.
Understanding Mounjaro's Week-by-Week Dose Schedule
Mounjaro dosing is designed as a stepped programme, with doses increasing every 4 weeks minimum. Unlike some medications that start at a full dose, tirzepatide starts low and builds gradually — this approach reduces side effects and allows your body to adjust. Here's the standard UK prescription timeline:
- Weeks 1–4: 2.5 mg once weekly (starting dose)
- Weeks 5–8: 5 mg once weekly (first escalation)
- Weeks 9–12: 7.5 mg once weekly (second escalation)
- Weeks 13–16: 10 mg once weekly (third escalation)
- Weeks 17–20: 12.5 mg once weekly (fourth escalation)
- Week 21 onwards: 15 mg once weekly (maximum maintenance dose)
Your prescriber may adjust this timeline based on how well you tolerate each dose. If side effects are significant, you may stay on a lower dose longer before escalating. The minimum time between dose increases is 4 weeks — never skip steps or increase faster. Each KwikPen contains 4 weeks of doses, so you'll need a new prescription every month.
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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.
Side Effects and What to Expect Week by Week
Side effects are most common when you first start or escalate the dose. Most people experience nausea, constipation, or diarrhoea in the first week or two, which often improves within 3–7 days as your body adjusts. Gastrointestinal symptoms are the most frequent — stomach pain, indigestion, and reduced appetite are typical in weeks 1–4. By week 5, many people report that initial nausea has settled, though appetite suppression continues (which is the intended effect). Always report severe or persistent symptoms to your GP or prescriber. Use our free GLP-1 weight loss calculator to estimate potential results and discuss realistic timelines with your prescriber, and track your progress weekly using our progress tracker to monitor both side effects and any weight changes.
- 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.
When do I start seeing weight loss results on Mounjaro?
Weight loss typically becomes noticeable in weeks 4–8 as your dose reaches 5 mg, though individual timelines vary significantly. Most people see gradual weight loss rather than rapid loss. The full effect of each dose is often seen after 4 weeks at that level. Clinical trials showed average weight loss of 15–21% body weight, but this is over the full treatment period (usually 1–2 years), not within the first few weeks.
Why does Mounjaro increase so slowly over weeks?
Gradual dose escalation minimises side effects and allows your body to adapt to each level. Starting at 2.5 mg and increasing every 4 weeks gives your system time to adjust before moving higher. This approach also helps your prescriber identify which dose works best for you personally. Faster escalation would likely cause more severe nausea and gastrointestinal upset.
Can I skip a week or delay my dose during the first month?
No — you should take your injection on the same day each week, every week. Missing doses or delaying more than a few days disrupts the dosing schedule and may affect results. If you accidentally miss a dose, take it within 4 days of the scheduled date, then resume your normal weekly schedule. More than 4 days late means you skip that dose.
What if I get severe nausea in week 1 — does it mean Mounjaro isn't right for me?
Nausea in week 1 is very common and usually settles within a few days to a week as your body adjusts. It doesn't necessarily mean you should stop. Most people continue and find nausea improves significantly by week 3–4. However, if symptoms are truly severe or you vomit persistently, contact your prescriber immediately — they may adjust your dose timing or recommend additional support.
When should I expect appetite suppression to start?
Appetite reduction typically begins within days of your first injection, even at the 2.5 mg starting dose. Many people notice they feel fuller after smaller meals by week 2–3. This appetite suppression is the primary mechanism that helps with weight management and generally continues and strengthens as your dose increases over the following weeks.
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.