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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What Are the Long-Term Side Effects of Tirzepatide?
Tirzepatide, the active ingredient in Mounjaro, is a prescription-only weekly weight loss injection that works as a dual GIP and GLP-1 receptor agonist. While clinical trials have shown it effective for weight management in the UK, questions about long-term safety are natural. This guide covers what we know about tirzepatide's long-term side effects, based on available clinical evidence and real-world data. Most side effects are short-term digestive issues, but some effects may continue with ongoing use. Always discuss any concerns with your GP or the prescriber managing your treatment.
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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.
Long-Term Side Effects from Clinical Evidence
Clinical trials for tirzepatide, including the SURMOUNT studies used to support UK licensing, ran for up to 72 weeks. However, longer-term safety data is still emerging. Based on available evidence, the main persistent side effects with continued use include:
- Nausea and appetite suppression — Often improves over the first 4–8 weeks but may persist in some users at lower intensity
- Constipation or diarrhoea — Gastrointestinal effects may continue as long as the medication is used, though many people adapt
- Stomach pain or discomfort — Some users report ongoing mild abdominal symptoms
- Hair loss (alopecia) — Reported in clinical trials; mechanism unclear but may relate to gradual weight management rather than tirzepatide itself
- Fatigue — Some users report persistent tiredness, particularly during dose increases
Side effects typically stabilise after the first 2–3 months. Most people find digestive symptoms become manageable or improve with time on a stable 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
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Safety Monitoring and When to Contact Your GP
Because tirzepatide is a relatively new weight loss medication in the UK, long-term safety data beyond 2–3 years is limited. The MHRA (UK medicines regulator) has placed tirzepatide under additional monitoring (Black Triangle ▼ status), meaning adverse effects are reviewed continuously. If you experience any of the following while taking tirzepatide, contact your GP immediately: severe, persistent stomach pain spreading to the back (possible pancreatitis), changes in vision, signs of gallbladder problems (upper stomach pain), serious allergic reactions, or unusual changes in heart rate. Your prescriber may adjust your dose, slow your escalation, or suggest stopping the medication if side effects become intolerable. Never stop abruptly without medical guidance — your appetite may return suddenly.
- 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.
Does tirzepatide cause permanent side effects?
No. Most side effects of tirzepatide are reversible. Digestive issues typically improve or resolve after stopping the medication. Hair loss is temporary and related to gradual weight management rather than tirzepatide itself. Always discuss concerns with your prescriber, who can adjust your dose or suggest alternatives if side effects persist.
How long does it take for tirzepatide side effects to go away?
Most digestive side effects improve within 2–8 weeks as your body adjusts. However, some people experience mild nausea or constipation for longer. After stopping tirzepatide, side effects usually resolve within 1–2 weeks. Individual timelines vary significantly — your GP can advise on your specific situation.
Is tirzepatide safe for long-term use beyond one year?
Clinical trials ran up to 72 weeks, showing it is safe in that timeframe. However, long-term safety data beyond 2–3 years is still emerging because tirzepatide is relatively new. The MHRA monitors safety continuously. Your prescriber will assess whether ongoing use remains appropriate for you based on your health and response.
Can tirzepatide cause pancreatitis or gallbladder problems long-term?
Pancreatitis and gallbladder issues are rare but serious. gradual weight management itself (from any cause) can increase gallstone risk. If you experience severe, persistent stomach pain spreading to your back, seek immediate medical attention. Your GP may order imaging to rule out pancreatitis or gallbladder disease.
Does tirzepatide affect metabolism long-term after stopping?
Tirzepatide does not permanently change your metabolism. Once you stop the medication, your appetite hormones return to baseline over 1–2 weeks. Weight management after stopping requires returning to dietary and exercise habits. Our GLP-1 weight loss calculator can help you plan realistic goals and track your progress throughout your treatment journey.
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.