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
Loss of Period During Weight Loss — What You Need to Know
Weight loss can sometimes affect your menstrual cycle, and in some cases, cause your period to stop temporarily or become irregular. This is more common than you might think, particularly with gradual weight management or very low-calorie diets. Understanding why this happens, when it's a concern, and what you can do about it is important for your health. If you're experiencing loss of period alongside weight management, it's essential to speak with your GP, who can assess your individual situation and advise on safe approaches to weight loss in the UK.
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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.
Why Does Weight Loss Affect Your Period?
Your menstrual cycle is regulated by hormones, including oestrogen and progesterone. When you lose weight—especially rapidly—your body's fat stores decrease, which can disrupt hormone production because fat cells produce oestrogen. This hormonal shift may cause your period to become irregular or stop temporarily, a condition sometimes called amenorrhoea.
Several factors increase this risk:
- Very gradual weight management (more than 1.5 kg per week)
- Extreme calorie restriction (below 800 kcal per day)
- Low body fat percentage (below 15–20%)
- High physical activity combined with low calorie intake
- Stress and lack of sleep during dieting
The NHS recommends losing weight at a safe, sustainable rate of 0.5–1 kg per week, which is less likely to disrupt your cycle. Our free weight loss calculator can help you understand healthy weight loss projections with prescription weight management approaches.
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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.
When Should You Seek Medical Advice?
If you've missed three or more consecutive periods during or after weight loss, contact your GP. Loss of period can indicate your body is under stress or not receiving enough nutrients to maintain normal hormone function. Your GP can:
- Rule out other causes (thyroid issues, PCOS, pregnancy)
- Check your nutritional status and bone health
- Advise on safe weight loss strategies
- Refer you to specialist services if needed
Prolonged amenorrhoea can increase your risk of bone loss and other health complications, so medical assessment is important. If you're using prescription weight loss medication like Wegovy (semaglutide) or considering GLP-1 weight loss options, your prescriber should be aware of menstrual changes, as they can adjust your approach if needed.
- 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.
Is it normal to lose your period when losing weight?
Yes, it's relatively common, especially with gradual weight management. Your menstrual cycle depends on hormone levels, and significant fat loss can disrupt oestrogen production. However, if your period stops, you should contact your GP to rule out other causes and ensure your weight loss rate is safe and sustainable.
Will my period come back if I regain the weight?
Often yes. In many cases, when weight stabilises at a healthier level and hormones rebalance, menstrual cycles resume. However, this can take several months. Your GP can monitor this and offer support if your cycle doesn't return within 3–6 months.
Can prescription weight loss medication affect my period?
GLP-1 medications like Wegovy may indirectly affect your cycle if they cause gradual weight management, but amenorrhoea is not a direct side effect. Your prescriber will assess your individual health before treatment. Always inform them of any menstrual changes during weight loss medication.
What's the safest rate of weight loss to protect my period?
The NHS recommends losing 0.5–1 kg per week through a moderate calorie deficit and physical activity. This rate is less likely to disrupt your hormones. Avoiding very low-calorie diets and extreme exercise without adequate nutrition helps protect your menstrual health.
Should I stop trying to lose weight if I lose my period?
No, but you should seek GP advice immediately. Your doctor can help you adjust your weight loss approach to a safer, more sustainable pace. Sustainable weight management through prescription weight loss options, combined with balanced nutrition and moderate activity, may be safer for your hormonal health than extreme dietary restriction.
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.