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 After 30 Days on Mounjaro?
When you start Mounjaro, the first 30 days represents your initial adjustment period at the starting dose of 2.5 mg. Most people experience noticeable changes during this time, though weight loss patterns vary significantly between individuals. Understanding what's typical after your first month can help you manage expectations and recognise when effects are normal versus when to contact your GP or prescriber.
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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.
Changes and Effects During Your First 30 Days
After 30 days on the starting dose of 2.5 mg tirzepatide (Mounjaro's active ingredient), most people report some noticeable effects. Appetite suppression typically begins within days for many users, with reduced food cravings and feeling fuller after smaller meals. This is the dual GIP and GLP-1 mechanism at work — activating two natural hormone pathways that regulate hunger and satiety.
Gastrointestinal side effects are most common during this initial phase. Nausea is reported by over 1 in 10 people, often peaking in the first 1–2 weeks then improving. Constipation and diarrhoea can alternate. These typically settle as your body adjusts, though remaining on the 2.5 mg dose for the full 4 weeks is important before increasing.
Weight loss varies widely at the starting dose. Some people lose 1–2 kg in the first month, others see minimal change. This is completely normal — clinical trials show significant results build over months, particularly as you progress to maintenance doses (5 mg and above). Your prescriber will assess your response and schedule your dose increase at week 5.
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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.
Managing Side Effects and Setting Realistic Expectations
If nausea develops, eating smaller, blander meals — avoiding fatty or greasy foods — often helps. Stay hydrated and eat slowly. Ginger tea or peppermint may provide relief. Most side effects improve by week 2–3 as your body adjusts, but contact your prescriber if they're severe or affecting your ability to eat adequately.
For weight loss expectations: losing 1–2 kg in 30 days is reasonable, but some people see none at this starting dose. This doesn't mean Mounjaro isn't working — the dose is deliberately low to let your body adjust. Clinical trials (SURMOUNT-1) showed average weight loss of around 15–21% over 72 weeks, but this builds gradually as doses increase. Use our GLP-1 weight loss calculator to estimate potential weight loss at different doses, and track your progress using our free progress tracker. Remember that individual results vary significantly.
- 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.
Is nausea in the first 30 days of Mounjaro normal?
Yes, nausea is very common during the first few weeks and affects more than 1 in 10 people starting Mounjaro. It typically peaks in week 1–2 then improves as your body adjusts to the 2.5 mg starting dose. Eating smaller meals, avoiding fatty foods, and staying hydrated usually help. Contact your prescriber if nausea is severe or prevents you eating adequately.
How much weight should I lose in the first 30 days on Mounjaro?
Weight loss varies widely at the 2.5 mg starting dose. Some people lose 1–2 kg, others lose none in the first month — both are normal. The 2.5 mg dose is deliberately low to allow adjustment. Significant weight loss typically builds as your dose increases to 5 mg and beyond over subsequent weeks and months.
When do I increase my Mounjaro dose after 30 days?
Your dose increases at week 5 (after 4 weeks on 2.5 mg) to 5 mg. Your prescriber will confirm this timeline at your initial assessment. You must remain on each dose for a minimum of 4 weeks before escalating, allowing your body to adjust and your prescriber to assess your response.
What's the difference between Mounjaro and Wegovy after 30 days?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist starting at 2.5 mg, whilst Wegovy (semaglutide) is a single GLP-1 agonist starting at 0.25 mg. Both are once-weekly injections, but Mounjaro's dual mechanism may produce different effects. Clinical trials show Mounjaro achieved slightly higher average weight loss (around 20.9% vs 14.9%), though individual responses vary. Your prescriber will recommend the most suitable option based on your health.
Should I see weight loss by 30 days or is it too early?
30 days is still very early in treatment — many people see minimal weight loss at the 2.5 mg starting dose. This is expected and doesn't indicate failure. Weight loss builds over months as your dose increases to 5 mg, 10 mg, and beyond. Check your BMI using our free BMI calculator to establish a baseline, then track changes over 8–12 weeks when you'll be on a higher, more effective dose.
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.