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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Weight Loss Injections Side Effects: Long-Term Safety in the UK
Weight loss injections like Wegovy and Mounjaro are prescription-only GLP-1 medications that may help reduce appetite and support weight management. However, understanding potential long-term side effects is important before starting treatment. Most side effects occur early in treatment, but some people experience ongoing effects or develop new ones as their body adjusts. This guide covers what long-term safety data shows, how side effects typically evolve, and when to speak with your GP about concerns. Always remember that individual responses vary significantly, and your prescriber can provide personalised guidance based on your health.
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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.
What the Long-Term Data Shows About Weight Loss Injections
Clinical trials for Wegovy (semaglutide) and Mounjaro (tirzepatide) lasted 68–72 weeks, providing valuable long-term safety data. The STEP-1 trial found that most gastrointestinal side effects — nausea, vomiting, constipation, and diarrhoea — were most common in the first 4–8 weeks as the body adjusted to the medication. By week 12, many people reported side effects becoming less severe or resolving entirely. However, some participants continued to experience mild nausea or digestive changes throughout the trial period.
Hair loss (alopecia) emerged as an important long-term observation — this occurred in some users and was typically linked to gradual weight management rather than the medication itself. gradual weight management can trigger temporary hair shedding (telogen effluvium). When weight loss stabilises, hair growth typically normalises.
No serious new side effects emerged in the 18+ month follow-up data, suggesting that prescription weight loss injections are generally well-tolerated over extended periods when used under medical supervision.
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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.
Long-Term Monitoring and When to Contact Your GP
If you're using weight loss injections, regular GP check-ups are important — your prescriber should monitor your blood sugar, cholesterol, and overall health during treatment. Some side effects require medical attention even if they're not emergencies: persistent nausea affecting nutrition, signs of dehydration, gallbladder pain, or unexplained changes in vision should always be reported to your GP.
Most people find that ongoing side effects become manageable with time. Keep a simple progress tracker to log how you feel week to week — this helps you spot patterns and communicate changes to your healthcare provider. Using our GLP-1 weight loss calculator alongside your GP's guidance can help you set realistic expectations and monitor progress throughout your treatment journey.
- 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.
Do weight loss injection side effects get better over time?
Yes, for most people. Gastrointestinal side effects like nausea and vomiting typically peak in the first 4–8 weeks, then improve as your body adjusts. Clinical trial data showed that by week 12, many participants experienced significant improvement. However, some people continue mild symptoms throughout treatment. Your GP can advise if adjustments are needed.
Is hair loss from weight loss injections permanent?
Hair loss associated with weight loss injections is usually temporary and related to gradual weight management, not the medication itself. This type of hair shedding (telogen effluvium) typically resolves as weight stabilises. Eating adequate protein and maintaining proper nutrition supports healthy hair growth during weight loss treatment.
Can you stay on weight loss injections long-term?
Weight loss injections like Wegovy can be used long-term under medical supervision for weight management and maintenance. However, prescribers regularly review whether continued treatment remains appropriate for your health goals. NHS guidance recommends ongoing monitoring through specialist services. Your GP or prescriber will assess this individually.
What are the most common long-term side effects reported?
The most frequently reported ongoing side effects are mild nausea, constipation, and occasional indigestion. Most are manageable with dietary adjustments and increased water intake. Serious long-term side effects are rare when used under medical supervision. Always report persistent symptoms to your GP for personalised advice.
Should I stop weight loss injections if side effects continue?
Do not stop prescription medication without medical guidance. If side effects persist or are affecting your quality of life, contact your GP or prescriber — they may adjust your dose, change your injection technique, suggest dietary strategies, or discuss alternatives. Your health and safety are the priority.
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.