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
How Long After Injecting Mounjaro Do Side Effects Start?
Side effects from Mounjaro (tirzepatide) typically begin within the first few hours to days after your injection, though the exact timing varies between individuals. Most people experience nausea, the most common side effect, within 24 hours of their first dose or after dose increases. Understanding this timeline helps you prepare for what to expect. Our GLP-1 weight loss calculator can help you explore how Mounjaro works alongside lifestyle changes at different doses, while the BMI calculator on this page allows you to track your health context throughout your weight management 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.
When Mounjaro Side Effects Typically Appear
Mounjaro side effects follow a fairly predictable pattern, though individual experiences vary significantly. Within the first 24 hours after injection, nausea is most common—affecting up to 1 in 3 people on higher doses. Days 2–4 often see gastrointestinal effects peak, including diarrhoea, constipation, and abdominal discomfort, as the medication begins working on your gut hormones. By week 2–3, many people report these effects diminishing as their body adapts, though some experience them for longer. Injection site reactions—redness, bruising, or mild pain—appear immediately but are usually mild and resolve within hours. The timing of symptom onset can differ after dose increases: moving from 5mg to 10mg may trigger a new wave of nausea lasting 2–5 days. Unlike some medications, Mounjaro side effects do not typically worsen over time; most adapt within the first 2–3 weeks of starting or increasing their dose.
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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.
Mounjaro vs Wegovy: Timeline and Mechanism Differences
While both Mounjaro and Wegovy cause similar side effects—nausea, diarrhoea, constipation—their onset timing can differ slightly due to different mechanisms. Wegovy (semaglutide) is a single GLP-1 agonist, whereas Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways. Clinical trial data (SURMOUNT for Mounjaro, STEP for Wegovy) shows both have comparable side effect profiles, but the dual mechanism may cause slightly more nausea in early weeks for some users. Wegovy's longer clinical history means more published patient data on timeline expectations. Both require prescription and clinical assessment—your GP or prescriber will discuss which suits your individual health profile, medical history, and tolerance for side effects. Neither medication is superior; suitability depends on your personal circumstances and how your body responds.
- 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.
Can I reduce Mounjaro side effects by injecting more slowly or splitting the dose?
No. Mounjaro is provided in pre-filled KwikPens with fixed doses that cannot be split. The entire dose is injected as designed. If side effects are severe, your prescriber may delay dose increases or temporarily reduce your dose, but this must be discussed with them. Never alter your injection technique without guidance.
Are Mounjaro side effects worse after the first injection or after dose increases?
Side effects can occur after your first injection, but many people experience them more noticeably after dose increases. You may feel fine at 2.5mg but develop nausea when moving to 5mg. This is normal and usually temporary—most people adapt within 1–3 weeks of each new dose.
How does Mounjaro's side effect timeline compare to other weight loss injections?
Saxenda (liraglutide) is a daily injection and may cause side effects with each dose, whereas Mounjaro is once-weekly. Wegovy (semaglutide) is also once-weekly with a similar timeline. All three have comparable side effect profiles; onset timing depends more on individual factors than the medication choice.
What should I do if Mounjaro side effects haven't stopped after 3 weeks?
Contact your prescriber. While most side effects settle within 2–3 weeks, some people experience prolonged symptoms. Your prescriber may investigate other causes, pause dose escalation, or consider whether Mounjaro is suitable for you. Never stop taking Mounjaro without guidance.
Does the Mounjaro KwikPen injection site affect when side effects start?
No. Whether you inject into your stomach, thigh, or arm does not change when systemic side effects begin. The medication enters your bloodstream within hours regardless of injection site. Local reactions (redness, bruising) at the injection site are separate and typically resolve quickly.
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.