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
Semaglutide Weight Loss Side Effects — What You Need to Know
Semaglutide is a GLP-1 receptor agonist used for weight management through Wegovy, a prescription-only weekly weight loss injection. Side effects are common when starting treatment, but most are temporary and tend to improve over time. Understanding what to expect helps you make an informed decision with your GP or healthcare provider about whether this medication is right for you. Use our GLP-1 weight loss calculator to explore potential outcomes, and consult your GP about individual side effect risks based on your health profile.
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 and NHS Context
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, and constipation — often experienced in the first few weeks as your body adjusts. Stomach pain and indigestion are also frequently reported. The NHS notes these effects typically peak during dose escalation and often settle as treatment continues.
How common are they? Based on the STEP clinical trials (the evidence used by NICE and MHRA to approve Wegovy), nausea affects approximately 25–40% of users at therapeutic doses, while gastrointestinal symptoms are reported by up to 60% of participants. These rates are significantly higher than placebo, making them the primary reason some people discontinue treatment.
- Nausea usually improves after 2–4 weeks on each dose
- Staying hydrated and eating smaller meals can help manage symptoms
- Your prescriber may recommend temporarily delaying dose increases if side effects are severe
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.
Getting Semaglutide Through the NHS vs Private Route
On the NHS, semaglutide (Wegovy) is only available through specialist tier 3 weight management services — your GP cannot prescribe it directly. You'll need referral and assessment, which can involve significant waiting times depending on your region. NICE has approved Wegovy for use within NHS services, but availability varies by integrated care board.
Privately, prescription assessment is faster (often within days), though you'll pay for the medication. Your prescriber will assess side effect risk during the clinical consultation before issuing a prescription. Whichever route you choose, discuss side effect concerns with your healthcare provider before starting. Track your response using our free progress tracker to monitor how side effects change over time and whether they improve with continued use.
- 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 semaglutide side effects last?
Most gastrointestinal side effects improve significantly within 2–4 weeks on each dose as your body adjusts. Some people experience improvement within days. However, nausea can recur when doses increase during the escalation phase. After reaching your maintenance dose, side effects typically stabilise or reduce further. If side effects persist beyond 8 weeks on a stable dose, discuss with your prescriber.
Is nausea the most common side effect with semaglutide?
Yes. Clinical trial data shows nausea is the most frequently reported side effect, affecting 25–40% of users at therapeutic doses, compared to 6% on placebo. It's usually mild to moderate and improves over time. Taking anti-nausea strategies like eating smaller meals, staying hydrated, and avoiding fatty foods can help manage symptoms effectively.
Can you still take semaglutide if you have IBS or digestive issues?
Semaglutide may not be suitable if you have severe gastroparesis (delayed stomach emptying) or severe digestion problems. Tell your GP or prescriber about any existing digestive conditions during assessment. If you have mild IBS, semaglutide may be tolerated, but your healthcare provider needs to assess individual risk before prescribing.
What serious side effects should I watch for with semaglutide?
Seek immediate medical attention for severe stomach pain (possible pancreatitis), serious allergic reactions (facial swelling, difficulty breathing), changes in vision, or persistent vomiting. These are rare but require urgent assessment. Always contact your GP or NHS 111 if you're unsure whether a symptom is serious — don't wait.
Does semaglutide cause hair loss?
Hair loss is not listed as a common side effect in STEP trials. However, gradual weight management (including from semaglutide) can trigger telogen effluvium — temporary hair shedding caused by nutritional stress during weight loss. This typically resolves within 3–6 months. Ensure adequate protein intake and discuss concerns with your GP if you notice significant hair loss.
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.