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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Tirzepatide Long-Term Side Effects: What the Evidence Shows
Tirzepatide, the active ingredient in Mounjaro, is a GIP and GLP-1 receptor agonist prescribed for weight management in UK adults. Understanding long-term side effects is important when considering this weekly injection. This guide compares what clinical trials revealed about tirzepatide versus Wegovy (semaglutide), helping you make an informed decision alongside your GP or prescriber. Individual experiences vary significantly, and only a qualified healthcare professional can advise on suitability for your circumstances.
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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.
How Tirzepatide's Mechanism Affects Long-Term Side Effects
Tirzepatide differs from other GLP-1 medications because it activates two pathways: the GIP receptor and the GLP-1 receptor. This dual mechanism may explain some differences in long-term side effect profiles compared to single-action GLP-1 receptor agonists like Wegovy.
The SURMOUNT-1 clinical trial, which tested tirzepatide up to 15 mg weekly, found that gastrointestinal side effects (nausea, vomiting, diarrhoea, constipation) were most common during the dose escalation phase—roughly weeks 1–16. For many participants, these effects declined or stabilised as the body adjusted to the medication. Importantly, the trial lasted 72 weeks, providing longer-term safety data than many weight loss medication studies.
- Most GI side effects occur during dose increases, not at maintenance doses
- Hair loss (alopecia) was reported in 3–4% of participants—an emerging concern monitored across all GLP-1 medications
- No new safety signals emerged after the 16-week escalation period in the SURMOUNT trial
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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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Tirzepatide vs Wegovy: Side Effect Comparison and Clinical Context
Both tirzepatide (Mounjaro) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK. STEP-1 trial data for Wegovy showed similar gastrointestinal side effects to SURMOUNT data for tirzepatide—nausea, diarrhoea, and vomiting were the most frequently reported. Neither medication carries a significantly safer long-term profile; the choice depends on individual factors assessed by your prescriber.
Key clinical differences: Tirzepatide has a dual GIP and GLP-1 mechanism; Wegovy works via GLP-1 only. Long-term data (beyond 72 weeks) for both medications is still being collected. Use our GLP-1 weight loss calculator to understand potential outcomes at different doses, and discuss long-term monitoring with your prescriber.
- 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.
What are the most common long-term side effects of tirzepatide (Mounjaro)?
Nausea, diarrhoea, vomiting, and constipation are most frequent. Hair loss (alopecia) has been reported in clinical trials. Most GI symptoms peak during the first 4–16 weeks of dose escalation and often improve as your body adapts. Always report persistent side effects to your prescriber.
Is tirzepatide safer than Wegovy long-term?
Clinical trial data does not suggest one is significantly safer than the other. SURMOUNT-1 (tirzepatide) and STEP-1 (Wegovy) both showed similar gastrointestinal side effect profiles. Your prescriber will assess which is more suitable based on your individual health, medical history, and other medications.
How long do tirzepatide side effects last?
Most side effects peak during dose escalation (weeks 1–16) and may improve at maintenance doses. Some people experience ongoing mild nausea or digestive changes. The SURMOUNT trial lasted 72 weeks; longer-term data beyond this is still being collected. Keep your prescriber informed of any persistent symptoms.
Can tirzepatide cause permanent side effects?
No permanent side effects have been identified in clinical trials, though long-term data collection is ongoing. Once tirzepatide is stopped, side effects typically resolve within a few days to weeks. However, weight regain may occur if lifestyle changes are not maintained.
What should I do if I experience side effects on tirzepatide?
Report any side effects to your GP or prescriber immediately. Some can be managed with dietary changes (smaller meals, avoiding fatty foods). Your prescriber may adjust your dose, delay escalation, or switch you to an alternative medication if side effects are intolerable. Never stop without medical guidance.
Ready to take control of your weight loss journey?
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