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
No Weight Loss at Week 3 on Mounjaro — Is This Normal?
It's completely normal to see no weight loss at week 3 of Mounjaro treatment. You're likely still on the starting dose (2.5 mg), which is designed for tolerance and adjustment rather than maximum weight loss effect. Most people don't begin seeing meaningful weight loss until weeks 5–8 when doses are escalated. Weight loss with Mounjaro is a gradual process that works best alongside a reduced-calorie diet and increased physical activity. If you have concerns, always speak with your GP or the prescriber who assessed you.
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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.
How Mounjaro Dosing Works in the First Weeks
Mounjaro uses a gradual dose escalation schedule because starting at the full dose would cause severe side effects. Here's the typical UK timeline:
- Weeks 1–4: 2.5 mg (starting/adjustment dose) — your body adapts to the medication
- Weeks 5–8: 5 mg (first maintenance dose) — weight loss typically becomes more noticeable
- Weeks 9–12: 7.5 mg (escalation) — effects usually increase
- Weeks 13+: 10 mg to 15 mg (higher maintenance doses)
The SURMOUNT-1 clinical trial showed that participants lost an average of 15% body weight on 5 mg and 19.5% on 10 mg — but these results were measured at 72 weeks, not 3 weeks. At week 3, you're still in the dose tolerance phase. Weight loss happens gradually and varies between individuals.
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.
Why Your Body Needs Time to Respond
Mounjaro (tirzepatide) works as a dual GIP and GLP-1 receptor agonist, which means it activates two hormone pathways to help regulate appetite and food intake. However, the medication needs time to build up in your system and for your body to adapt. Side effects like nausea often appear before weight loss does — this is normal and typically improves as your body adjusts. Additionally, sustainable weight loss requires consistent dietary changes and increased activity. If you're not in a calorie deficit, medication alone won't produce results. Use our free GLP-1 weight loss calculator to estimate realistic projections at your current dose, and our BMI calculator to track your baseline. These tools help you monitor progress over weeks and months, not days.
- 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.
Should I be worried if I haven't lost weight in 3 weeks on Mounjaro?
No. Three weeks is too early to expect significant weight loss. You're still on the 2.5 mg starting dose, which prioritises tolerance over maximum effect. Most people see results after the first dose escalation (week 5 onwards). If you have health concerns, contact your GP or prescriber for reassurance.
When will I start seeing weight loss on Mounjaro?
Weight loss typically becomes noticeable between weeks 5–12 as doses are escalated and your body adapts. Some people see results earlier, others later. Individual responses vary significantly. Consistent dietary changes and physical activity are essential — medication works best alongside these lifestyle changes.
Is it normal to have side effects but no weight loss yet?
Yes, absolutely normal. Nausea, constipation, and reduced appetite often appear in the first 2–4 weeks as your body adjusts. Weight loss typically follows as side effects settle and doses increase. If side effects are severe, contact your prescriber — they may adjust your approach.
What if I'm still not losing weight after 8 weeks on Mounjaro?
After 8 weeks, you should see some weight loss if diet and activity are consistent. If not, discuss with your GP or prescriber. Possible factors include: insufficient calorie deficit, medical conditions affecting metabolism, or medication tolerance. Your prescriber can assess whether a dose increase is appropriate or other adjustments are needed.
How much weight should I lose on Mounjaro per week?
The NHS recommends a sustainable loss of 0.5–1 kg per week, though this varies. Mounjaro may help achieve or exceed this, but results depend on calorie deficit, individual metabolism, and adherence. Clinical trials showed average loss of 15–21% body weight over 72 weeks. Track your progress using our free progress tracker over weeks and months, not days or single 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.