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 With Mounjaro in Week 1?
Most people do not experience significant weight loss in week 1 of Mounjaro. The first dose (2.5 mg) is a starting dose designed to help your body adjust to the medication, not to deliver gradual weight management. Week 1 is typically when side effects like nausea or mild appetite changes become noticeable. Real weight loss usually begins after several weeks once your dose has been increased and your body has adapted. It's important to understand realistic expectations from the start of your Mounjaro journey.
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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.
Why Week 1 Weight Loss Is Usually Minimal
The 2.5 mg starting dose is not the therapeutic dose. Mounjaro uses a gradual escalation schedule, with the dose remaining at 2.5 mg for the first 4 weeks before increasing to 5 mg. Your prescriber follows this structured approach to reduce side effects and allow your body to adapt. During week 1, the medication begins activating GIP and GLP-1 receptors in your gut and brain, but the effect is still mild.
Weight loss requires several weeks of consistent dosing. Clinical trials showed that meaningful weight loss typically emerges after 8–12 weeks as the dose reaches therapeutic levels (usually 10 mg or higher). Any weight loss in week 1 is more likely to be water weight or natural daily fluctuation rather than fat loss. Individual results vary significantly—some people notice appetite suppression within days, whilst others take 2–3 weeks to feel any effect.
- Week 1–4: Adjustment phase at 2.5 mg
- Week 5–8: First escalation to 5 mg
- Week 9+: Further escalation toward therapeutic dose
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 You Might Notice in Week 1
During your first week of Mounjaro, you may experience gastrointestinal side effects such as nausea, mild indigestion, or loss of appetite. Some people report feeling unusually full after eating smaller portions—this is the GLP-1 mechanism starting to work. Others notice no change at all in week 1. Injection site reactions (redness or mild bruising) are common but usually resolve within hours. Use our GLP-1 weight loss calculator to project realistic weight loss over 12–24 weeks rather than week 1 alone. Keep a food diary and monitor how you feel; report any persistent side effects to your prescriber, especially severe stomach pain.
- 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 lose weight in the first week of Mounjaro?
Most people do not experience significant weight loss in week 1. The 2.5 mg starting dose is designed for adaptation, not gradual weight management. Any change in week 1 is usually water weight or daily fluctuation. Meaningful fat loss typically starts after 4–8 weeks as your dose increases toward therapeutic levels.
Is Mounjaro a weekly weight loss injection?
Yes, Mounjaro is a once-weekly prescription weight loss injection. Each dose is given the same day each week using a pre-filled KwikPen. You inject 2.5 mg in week 1, then the dose increases every 4 weeks if tolerated, up to a maximum of 15 mg weekly.
How does Mounjaro compare to Wegovy for week 1 results?
Both Mounjaro and Wegovy use gradual dose escalation, so week 1 results are similar—usually minimal weight loss. Wegovy starts at 0.25 mg (even lower than Mounjaro's 2.5 mg), making week 1 effects even milder. Clinical trials showed Mounjaro achieved slightly greater average weight loss overall (~20% vs ~15% body weight) at maximum dose, though individual results vary.
What should I do if I have severe nausea in week 1?
Mild nausea is common in week 1 and often settles as your body adjusts. Eat smaller, frequent meals and avoid fatty or rich foods. If nausea is severe or persistent, contact your prescriber immediately—they may adjust your approach. Never stop the medication without guidance.
Can I use the BMI calculator to track week 1 progress?
Yes, our free BMI calculator lets you check your BMI weekly. However, one week is too short to see meaningful BMI change. Track your weight using our progress tracker over 4–8 weeks for a clearer picture of how Mounjaro is working for you alongside your dose escalation.
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.