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 Weight Loss Injections
Weight loss injections are prescription-only medications that may help with weight management in the UK, but they do come with potential side effects. Most side effects are mild and temporary, particularly during the initial weeks of treatment, though some people experience more significant gastrointestinal effects. Understanding what to expect can help you make an informed decision alongside your GP or prescriber. This guide covers the most common side effects of GLP-1 weight loss medications used in the UK, how they typically develop, and what medical support is available if you experience them.
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.
Most Common Side Effects of Weight Loss Injections
The most frequently reported side effects of weight loss injections like Wegovy (semaglutide) and Mounjaro (tirzepatide) affect the digestive system. Nausea is the most common, occurring in more than 1 in 10 people starting treatment. Other prevalent side effects include diarrhoea, vomiting, constipation, and abdominal pain — these reflect how GLP-1 and GIP/GLP-1 medications slow gastric emptying (how quickly food leaves the stomach).
Many of these gastrointestinal effects tend to improve as your body adjusts to the medication, particularly over the first 4–8 weeks. Injection site reactions (redness, bruising, or tenderness) are also common but usually mild and resolve quickly. Less frequent but notable side effects include fatigue, dizziness, indigestion, headache, and hair loss (alopecia), which some users report during weight loss phases.
It's important to note that individual responses vary significantly — some people experience minimal side effects whilst others find gastrointestinal symptoms more bothersome. Your prescriber can discuss risk factors and ways to manage symptoms as you begin prescription weight loss treatment.
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 Medical Attention
Whilst most side effects are mild, certain symptoms require urgent medical attention. Seek immediate help if you experience severe, persistent abdominal pain that radiates to your back — this may indicate pancreatitis (pancreas inflammation). Similarly, signs of serious allergic reaction (facial swelling, difficulty breathing, throat tightness) need emergency care.
Other concerning symptoms include unexplained changes in vision (particularly relevant if you have type 2 diabetes, as weight loss medications may affect diabetic retinopathy), persistent vomiting preventing food intake, or severe dehydration. Pain in the upper right area of the abdomen may suggest gallbladder problems. If you experience any of these symptoms whilst using weight management UK injections, contact your GP, NHS 111, or attend A&E. Always report unusual symptoms to your prescriber at your next review — they can assess whether to continue, adjust the dose, or switch medications.
- 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 go away?
Most side effects, particularly nausea and digestive symptoms, improve within the first 4–8 weeks as your body adjusts. However, some people continue to experience mild symptoms throughout treatment. Your prescriber can advise on managing persistent effects or whether dose adjustments might help.
Can you take anti-nausea medication with weight loss injections?
Speak to your prescriber or pharmacist before taking any additional medication, as some anti-nausea drugs may interact with GLP-1 medications or affect how they work. Your prescriber may recommend dietary adjustments or other strategies first, such as eating smaller meals or avoiding fatty foods.
What's the difference between Wegovy and Mounjaro side effects?
Both medications cause similar gastrointestinal side effects (nausea, diarrhoea, constipation, stomach pain) as they affect digestion. Mounjaro uses a dual GIP/GLP-1 mechanism whilst Wegovy is GLP-1-only, but clinical experience shows comparable side effect profiles. Individual responses vary more than medication choice.
Should I stop taking weight loss injections if side effects are severe?
Never stop prescription medication without medical guidance. If side effects are severe, contact your GP or prescriber — they may reduce your dose, adjust your injection schedule, or explore alternative weight loss medications. Stopping abruptly without support may not be safe.
Can you use our GLP-1 calculator to estimate side effect likelihood?
Our free weight loss calculator helps project potential weight loss at different doses, which may help you discuss realistic expectations with your prescriber. However, side effect risk is individual and depends on personal health factors — your prescriber's assessment is essential before starting treatment.
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.