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 Mounjaro Injection Do Side Effects Start?
Side effects from a Mounjaro injection typically begin within the first few hours to a few days after your injection, though the timing varies significantly between individuals. Most people experience nausea or stomach discomfort as their first symptom, often within 24–48 hours of their weekly weight loss injection. The severity and duration depend on your dose, how your body responds, and whether you've taken the medication before. Understanding the typical timeline can help you prepare and manage any effects effectively whilst benefiting from this prescription-only GLP-1 medication.
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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 Onset Timeline
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, which means it activates two hormone pathways rather than one. This mechanism difference affects how quickly side effects emerge and their intensity.
Clinical data from the SURMOUNT trials shows that:
- Nausea and stomach upset typically start within 24–48 hours of injection and may peak at 48–72 hours
- Diarrhoea or constipation can appear within 2–3 days and may persist for several days
- Loss of appetite usually develops gradually over the first week
- Injection site reactions (redness, mild bruising) appear immediately but resolve within hours to days
Wegovy (semaglutide), a GLP-1 only agonist, shows a similar timeline in STEP trials, though some users report slightly milder nausea due to its single mechanism of action. Individual variation is substantial—some people experience minimal symptoms whilst others report more significant effects.
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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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Managing Side Effects and Dose Titration
Both Mounjaro and Wegovy use a gradual dose titration schedule specifically designed to reduce side effect severity. Starting at a low dose (Mounjaro 2.5 mg, Wegovy 0.25 mg) and increasing every 4 weeks gives your body time to adjust.
Most side effects peak in the first week after each dose increase, then gradually improve. Practical strategies include: eating smaller, frequent meals; staying hydrated; avoiding fatty or greasy foods; and taking the injection at a time when mild nausea is manageable (many people choose evening). If side effects become severe or persistent, your prescriber may pause dose escalation. Your GP or the prescriber conducting your clinical assessment can discuss which medication's side effect profile suits your circumstances best.
- 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.
How long do Mounjaro side effects typically last after injection?
Most common side effects (nausea, stomach discomfort) peak within 48–72 hours of injection and then gradually improve over 3–5 days. By day 7, many people feel back to normal. Diarrhoea or constipation may persist longer—sometimes 5–7 days. However, individual timelines vary significantly.
Are Mounjaro side effects worse than Wegovy's?
Clinical trials show Mounjaro may cause slightly more intense nausea initially because it activates two hormone pathways rather than one. However, this varies by individual. Some people tolerate Mounjaro better than Wegovy and vice versa. Your prescriber can help assess which is likely better suited to you based on your medical history.
Do side effects get better each week with Mounjaro?
Yes, typically. Once you're on a stable dose, side effects usually reduce significantly after the first injection or two at that dose level. When your dose increases (every 4 weeks), you may experience a brief increase in side effects, which then settle again. Using our free GLP-1 weight loss calculator can help you understand the dose progression timeline.
What should I do if side effects start immediately after my Mounjaro injection?
Contact your prescriber if you experience severe symptoms within the first few hours—though this is uncommon. Mild nausea within 24–48 hours is expected and usually improves without intervention. Eating light meals, staying hydrated, and resting help. Severe, persistent vomiting or abdominal pain warrants urgent medical advice.
Is the timeline different for the first Mounjaro injection versus later ones?
Many people report that their body "learns" to tolerate Mounjaro. The first injection at a new dose often triggers more noticeable side effects than subsequent injections at the same dose. This is why prescribers wait 4 weeks before increasing—your system adapts. Track your experience using our free progress tracker to identify patterns.
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.