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
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When Do Side Effects Start With Mounjaro vs Wegovy?
Side effects from Mounjaro (tirzepatide) typically begin within the first few days to first week of your initial injection, though timing varies between individuals. Wegovy (semaglutide) follows a similar pattern. Both medications work through different mechanisms—Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy activates GLP-1 only—but side effect onset is comparable. This guide explains when you're most likely to experience effects, how they compare, and what NHS guidance suggests about managing them during your dose titration period.
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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.
Side Effect Timing: Mounjaro vs Wegovy Clinical Evidence
Mounjaro side effect onset: Most people report nausea, the most common side effect, beginning within 24–48 hours of their first 2.5 mg injection. Gastrointestinal effects (nausea, diarrhoea, constipation, stomach pain) typically peak during weeks 1–2 and often improve within 2–3 weeks as the body adjusts. More severe effects like vomiting are less common and usually occur in the first week.
- Week 1–2: Nausea, loss of appetite, mild stomach discomfort most likely
- Week 2–4: Effects often peak then gradually reduce as tolerance builds
- Week 4+: Many people report significant improvement, especially if nausea management strategies are used
Wegovy side effect timing: Clinical trial data from STEP studies shows similar patterns—nausea typically emerges within days of the starting 0.25 mg dose, with gastrointestinal symptoms most pronounced in weeks 1–3. Because Wegovy has a slower escalation schedule (staying at each dose for 4 weeks), side effects may be more staggered across the titration period. The dual mechanism of Mounjaro may cause slightly faster onset of appetite suppression in some users, though individual variation is significant.
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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.
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Individual Factors and Prescriber Guidance
Your specific side effect timeline depends on several factors: body weight, stomach sensitivity, concurrent medications, and whether you're taking other GLP-1 medicines. Some people experience minimal side effects; others report significant nausea for 2–3 weeks. Your prescriber will discuss side effect management during your clinical assessment and may suggest taking Mounjaro with food, staying well hydrated, or adjusting timing of other oral medications. NHS guidance emphasises that side effects do not mean the medication isn't working—they're a sign the medication is active in your system. If side effects are severe or persistent beyond 4 weeks, contact your prescriber; dose adjustment or a switch to Wegovy may be appropriate. Always use our GLP-1 weight loss calculator to understand expected outcomes, and track your tolerance journey with our free progress tracker.
- 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.
Does Mounjaro cause side effects from the first injection?
Most people experience some nausea within 24–48 hours of their first 2.5 mg Mounjaro injection, though severity varies greatly. Some report no side effects; others experience mild nausea that resolves within days. Gastrointestinal effects (nausea, diarrhoea, constipation) are most common in weeks 1–2 and often improve as your body adapts.
Are side effects worse with Mounjaro than Wegovy?
Clinical trials show similar side effect profiles overall. Mounjaro's dual GIP+GLP-1 mechanism may cause faster appetite suppression, which some experience as more pronounced early nausea. Wegovy's slower escalation schedule may spread side effects over a longer period. Individual experience varies significantly—prescriber assessment determines which is most suitable for you.
How long do Mounjaro side effects typically last?
Most gastrointestinal side effects peak in weeks 1–2 and improve significantly by week 3–4 as your body adjusts. Some people experience symptoms for only 3–5 days; others report 2–3 weeks of mild nausea. Persistent severe side effects beyond 4 weeks warrant discussion with your prescriber about management strategies or dose adjustment.
Can I reduce my Mounjaro dose if side effects are severe?
Your prescriber may delay escalation to the next dose if side effects are problematic, but the 2.5 mg starting dose cannot be reduced further. If early side effects are severe, inform your prescriber immediately—they may discuss staying on 2.5 mg longer, adjusting timing, or switching to an alternative medication like Wegovy.
What should I do if side effects don't improve after 4 weeks?
Contact your prescriber. Persistent nausea, vomiting, or stomach pain after the first month warrants assessment. Management options include medication adjustments, anti-nausea support, dietary changes, or switching to Wegovy. Your GP or prescriber can provide personalised guidance based on your tolerance and health context.
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