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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Spotting on Mounjaro — Is It a Side Effect?
Spotting (light vaginal bleeding outside your normal cycle) is not listed as a common side effect of Mounjaro (tirzepatide) in the UK prescribing information. However, any unexpected vaginal bleeding should be reported to your GP promptly, as it may be unrelated to the medication and could indicate an underlying health concern. This guide explains what spotting is, when to seek advice, and how to distinguish between normal variations and symptoms that need medical attention.
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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 Clinical Evidence Shows About Mounjaro and Spotting
In the SURMOUNT clinical trials that led to Mounjaro's UK approval, spotting or abnormal vaginal bleeding was not reported as a very common or common side effect. The most frequently reported side effects were gastrointestinal — nausea, diarrhoea, vomiting, and constipation. However, individual responses to tirzepatide vary significantly.
Spotting can occur for many reasons unrelated to Mounjaro, including hormonal changes, stress, infections, contraceptive use, or underlying gynaecological conditions. If you experience spotting while taking Mounjaro, it is important not to assume it is caused by the medication without medical assessment. Always inform your GP of any new or unusual symptoms, especially bleeding. Your prescriber needs this information to determine whether it is related to the injection or requires separate investigation.
Some women report changes in their menstrual cycle while on GLP-1 medications, though this is not formally documented as a side effect. gradual weight management itself can sometimes affect hormone levels and cycle regularity.
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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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When to Contact Your GP About Spotting
Contact your GP if spotting is persistent, heavy, accompanied by pain, or if you experience unexpected bleeding between periods that is unlike your normal pattern. You should also seek advice if spotting occurs alongside other symptoms such as severe abdominal pain, fever, or signs of infection.
Your GP can assess whether the spotting is related to Mounjaro, your contraceptive method, hormonal changes from weight loss, or something else entirely. If you are concerned about whether to continue your Mounjaro prescription, your prescriber can discuss this with you at your next review. Never stop taking Mounjaro suddenly without medical guidance — discuss any concerns with your healthcare team first. Keep a brief note of when spotting occurs and any other symptoms to share with your GP.
- 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 spotting a known side effect of Mounjaro in the UK?
Spotting is not listed as a very common or common side effect in the UK prescribing information for Mounjaro. The most commonly reported side effects are gastrointestinal (nausea, diarrhoea, vomiting, constipation). If you experience spotting, it may be unrelated to the medication and should be evaluated by your GP.
Could my spotting be caused by Mounjaro dose titration?
Mounjaro dose titration itself is not known to cause spotting. However, gradual weight management from any weight loss medication can sometimes affect hormone levels and menstrual regularity. This varies between individuals. If spotting coincides with dose increases, mention this to your GP.
Should I stop taking Mounjaro if I start spotting?
Do not stop Mounjaro suddenly without medical advice. Contact your GP to discuss the spotting first. Your prescriber can assess whether it is related to the medication or requires separate investigation. Stopping suddenly may not be necessary and could affect your weight management plan.
How is spotting on Mounjaro different from spotting on other medications?
Spotting caused by hormonal contraceptives or other medications follows different patterns. If you are on oral contraceptives with Mounjaro, remember that Mounjaro may reduce contraceptive absorption — use additional barrier protection for 4 weeks after starting or increasing doses. Discuss with your GP or prescriber.
What should I tell my GP about spotting while taking Mounjaro?
Tell your GP when the spotting started, how long it lasts, whether it is heavier or lighter than normal, and any other symptoms (pain, fever, discharge changes). Mention your Mounjaro dose and how long you have been taking it. This helps your GP determine the cause and whether investigation is needed.
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