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
Side Effects of Tirzepatide — What to Expect
Tirzepatide is a prescription-only weight management injection licensed in the UK under the brand name Mounjaro. Like all medications, tirzepatide can cause side effects — though not everyone experiences them. Understanding what to expect helps you decide whether this GLP-1 and GIP receptor agonist is right for you. This guide covers the most common reactions, how long they typically last, and when to contact your GP. Remember, individual responses vary, and a clinical assessment is required before tirzepatide is prescribed.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
Common Side Effects of Tirzepatide
Most tirzepatide side effects are mild to moderate and often improve over time as your body adjusts to the injection. According to UK clinical data, the most frequently reported reactions include:
- Nausea — affects more than 1 in 10 people, especially in the first few weeks. Usually settles as you progress to a higher dose.
- Diarrhoea and constipation — digestive changes occur because tirzepatide slows how quickly food leaves your stomach. Eating smaller, regular meals can help.
- Vomiting — less common than nausea but can happen, particularly if you eat too much or too quickly.
- Stomach pain or indigestion — mild abdominal discomfort is common as your GIP and GLP-1 receptor systems adjust.
- Fatigue and dizziness — some users report feeling tired or lightheaded, especially in the first 1–2 weeks after starting.
- Hair loss (alopecia) — reported by some users; typically temporary and related to gradual weight management.
These side effects do not require emergency action but should be monitored. Use our GLP-1 weight loss calculator to understand potential outcomes and discuss side effect management with your GP or prescriber.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Serious Side Effects — When to Seek Help
Seek immediate medical attention if you experience:
- Severe stomach pain spreading to your back — may indicate pancreatitis, a serious pancreas inflammation requiring emergency care.
- Gallbladder problems — upper stomach pain, especially after meals, may suggest gallstone complications.
- Serious allergic reactions — swelling of face, lips, tongue, or throat; difficulty breathing; severe rash.
- Changes in vision — sudden vision changes, especially in people with diabetes, may require urgent assessment.
- Rapid or severe dehydration — extreme thirst, dark urine, dizziness from vomiting or diarrhoea.
These reactions are rare but serious. Contact your GP, NHS 111, or A&E immediately if you develop any of these symptoms. Always inform healthcare staff that you are taking tirzepatide.
- 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 tirzepatide side effects last?
Most side effects improve within 2–4 weeks as your body adjusts. Nausea and digestive changes often settle by week 3–4. Some users experience improvements when their dose increases, then mild symptoms return briefly before settling again. If side effects persist beyond 4–6 weeks, speak to your prescriber about dose adjustment.
Can I reduce tirzepatide side effects?
Yes. Eat smaller, more frequent meals; avoid fatty, greasy foods; stay hydrated; and eat slowly. Ginger tea or peppermint may help nausea. If side effects are severe, your prescriber may slow your dose escalation (staying on your current dose longer before increasing). Never stop tirzepatide suddenly without medical guidance.
Is tirzepatide the same as semaglutide in terms of side effects?
Tirzepatide (Mounjaro) is a dual GIP and GLP-1 receptor agonist, whilst semaglutide (Wegovy) is a GLP-1 agonist only. Tirzepatide may cause similar side effects to semaglutide but tends to produce greater weight loss in clinical trials. Side effect profiles are comparable, though individual reactions vary. Your prescriber can discuss which suits you best.
Does tirzepatide cause permanent hair loss?
Hair loss reported with tirzepatide is typically temporary and related to gradual weight management (telogen effluvium), not the medication itself. Hair usually regrows within 3–6 months as weight loss stabilises. If hair loss is severe or prolonged, discuss with your GP. Ensure adequate protein and micronutrient intake to support hair health.
Can I take paracetamol or other medicines with tirzepatide?
Tirzepatide may reduce how quickly your body absorbs oral medications because it slows stomach emptying. Space other oral medications at least 1 hour apart from tirzepatide. Oral contraceptives need additional barrier protection for 4 weeks after starting and after each dose increase. Always tell your prescriber about all medications before starting tirzepatide.
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.