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 Mounjaro Side Effects Start?
Mounjaro side effects typically begin within the first few days of starting the injection, with nausea and stomach discomfort being the most common early symptoms. Most people experience peak side effects during the first 2–4 weeks on each dose, though timing and severity vary significantly between individuals. Understanding when to expect these effects and how they compare to alternative medications like Wegovy can help you prepare and know what is normal. This guide covers the timeline, what to expect, and how Mounjaro's side effect pattern compares to other GLP-1 medications available in the UK.
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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.
Mounjaro vs Wegovy: Side Effect Timeline Comparison
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, which means it works on two hormone pathways rather than one. This dual mechanism may contribute to a different side effect profile compared to Wegovy (semaglutide), which activates the GLP-1 pathway only. In the SURMOUNT clinical trials, nausea affected over 25% of Mounjaro users at higher doses, with peak incidence occurring in the first 1–2 weeks after each dose increase. Most side effects, including nausea, diarrhoea, and vomiting, peak around weeks 1–4 on each new dose, then gradually improve as the body adjusts. Wegovy, based on the STEP trials, shows a similar pattern but with slightly different proportions — gastrointestinal side effects remain common but nausea may be less severe in some patients. Neither medication should cause side effects immediately upon injection; effects typically emerge over hours or days. Important note: Mounjaro requires a UK prescription following clinical assessment, as does Wegovy.
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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 When to Seek Medical Advice
Side effect onset and duration depend on individual factors including your starting dose, weight, diet, stomach sensitivity, and overall health. The Mounjaro KwikPen delivery system allows for gradual dose titration — starting at 2.5 mg and increasing by 2.5 mg every four weeks until reaching your effective dose — which helps your body adapt. Some people report minimal side effects, whilst others experience more significant symptoms in the first few weeks. Seek urgent medical advice if you develop severe, persistent stomach pain spreading to the back (possible pancreatitis), serious allergic reactions, or sudden changes in vision. Most common side effects resolve naturally within 2–3 weeks as your body adjusts. Your GP or prescriber can offer practical strategies to manage side effects, including dietary adjustments and taking your injection at times when you can rest if nausea occurs.
- 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.
Do Mounjaro side effects start immediately after injection?
No. Most Mounjaro side effects appear within hours or the first few days after injection, not immediately. Nausea and stomach discomfort typically peak 1–3 days after your weekly injection. Constipation or diarrhoea may develop over 2–3 days. Effects are usually mild initially but can intensify if you eat large meals or fatty foods. Individual timing varies.
How do Mounjaro and Wegovy side effect timings compare?
Both medications show similar patterns — peak side effects occur 1–2 weeks after starting or increasing dose. Mounjaro's dual GIP/GLP-1 mechanism may cause slightly different symptom profiles; some users report more nausea with Mounjaro, whilst others experience more diarrhoea with Wegovy. Neither pattern is universally "worse" — individual responses vary significantly. Your prescriber can help choose the best option for you.
Will Mounjaro side effects improve over time?
Yes. For most people, side effects improve noticeably after 2–3 weeks on the same dose. Nausea, the most common side effect, typically becomes milder as your body adjusts to the medication. When you increase to a higher dose, side effects may reappear briefly before settling again. This pattern repeats through each dose adjustment during Mounjaro dose titration.
What should I eat or avoid to reduce Mounjaro side effects?
Eat small, frequent meals rather than large ones. Avoid fatty, fried, or heavily spiced foods — these can trigger nausea and stomach pain. Stay hydrated with water and electrolyte drinks. Ginger tea or anti-nausea foods may help. Simple carbohydrates, lean proteins, and vegetables are often better tolerated. Your GP or dietitian can offer personalised dietary advice alongside your UK prescription.
When should I contact my GP about Mounjaro side effects?
Contact your GP if side effects are severe, persistent after 3 weeks, or if you develop severe stomach pain, persistent vomiting (preventing hydration), serious allergic reactions, or changes in vision. Mild nausea, diarrhoea, or stomach discomfort in the first 2 weeks are normal. Your prescriber can adjust your dose or provide management strategies to help you tolerate the medication.
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