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 Taking Mounjaro Do You Get Side Effects?
Side effects from Mounjaro (tirzepatide) typically begin within the first few days of your initial injection, though the timing and severity vary significantly between individuals. Most people experience their first side effects within 24 to 72 hours, with nausea and digestive symptoms being the most common. Understanding when side effects usually appear can help you prepare and know when to contact your GP if symptoms become concerning or persistent.
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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.
When Do Mounjaro Side Effects Usually Start?
Most people taking Mounjaro experience side effects within the first few days of their initial injection, typically between 24 and 72 hours. The most commonly reported early side effects are nausea, diarrhoea, and stomach discomfort.
Timing varies considerably depending on:
- Individual metabolism — how quickly your body processes the medication
- Dose strength — starting doses (2.5 mg) usually cause milder effects than higher maintenance doses
- Food intake — eating smaller, frequent meals may reduce nausea intensity
- Hydration status — dehydration can worsen nausea and digestive symptoms
Side effects tend to peak within the first two weeks after starting or increasing your dose, then gradually improve as your body adjusts. This adjustment period is sometimes called the "titration period," and it's why Mounjaro uses a gradual dose escalation schedule — allowing your system time to adapt at each level.
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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.
Side Effects That Appear Later — Weeks 2 to 4
While initial nausea and stomach symptoms often ease after the first two weeks, some people report new or persistent side effects emerging in weeks 2 to 4, including fatigue, headaches, and injection site reactions (redness or bruising).
Hair loss (alopecia) is a less common side effect that may not appear until several weeks into treatment, as it's linked to gradual weight management rather than the medication itself. If you're experiencing persistent symptoms beyond four weeks, contact your GP or prescriber — they can assess whether continuing at your current dose is appropriate or if an adjustment is needed. Try our free GLP-1 weight loss calculator to understand typical timelines and expected outcomes at different Mounjaro doses.
- 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 get Mounjaro side effects immediately after injection?
Yes. Side effects often begin within 24 to 72 hours of your first Mounjaro injection. Nausea is the most common early effect. However, some people experience minimal side effects initially, especially at the starting dose of 2.5 mg. Individual responses vary significantly based on metabolism and tolerance.
How long do Mounjaro side effects typically last?
Most side effects peak within the first two weeks and gradually improve as your body adjusts. For many people, nausea and digestive symptoms ease substantially after three to four weeks. However, some people experience persistent mild effects throughout treatment. This is why dose increases happen over 4+ weeks — to allow time for adjustment.
What should I do if Mounjaro side effects are severe?
Contact your GP or prescriber immediately if you experience severe stomach pain (possible pancreatitis), persistent vomiting, serious allergic reactions, or difficulty breathing. For moderate side effects, eat smaller meals, stay hydrated, and avoid high-fat foods. Your prescriber may recommend staying at your current dose longer before increasing.
Do Mounjaro side effects happen with every dose increase?
Often, yes. When you increase your dose (e.g., from 5 mg to 7.5 mg), you may experience side effects again as your body adjusts to the higher amount. This usually settles within two weeks. The gradual titration schedule (minimum four weeks at each dose) is designed to minimise this adjustment period impact.
Is there a difference between Mounjaro and Wegovy side effect timelines?
Yes. Mounjaro (tirzepatide) often causes side effects faster and potentially more intensely than Wegovy (semaglutide) because it activates two hormone pathways (GIP and GLP-1) rather than one. Wegovy's side effects typically develop within a similar timeframe but may be milder at comparable doses. Your prescriber can advise which is more suitable for you.
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.