Medical Weight Loss: What to Know About Wegovy and Mounjaro
Weight-loss injections such as Wegovy (semaglutide) and Mounjaro (tirzepatide) have transformed obesity treatment. Here is how they work, who they are for, the evidence behind them and how to use them safely.
Key takeaways
- Wegovy and Mounjaro are prescription-only medicines for weight management, used alongside a reduced-calorie diet and increased activity.
- In clinical trials, average weight loss was around 15% with semaglutide 2.4 mg and around 20% with the highest dose of tirzepatide.
- The most common side effects are nausea, diarrhoea, constipation and vomiting, usually when starting or increasing the dose.
- Only obtain these medicines with a prescription from a GPhC-registered pharmacy or regulated clinic; fake pens are in circulation.
Understanding obesity
Obesity is a long-term condition that raises the risk of type 2 diabetes, heart disease, some cancers, joint problems and sleep apnoea. Body mass index (BMI) is a starting point for assessing weight:
| BMI | Category |
|---|---|
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 39.9 | Obesity |
| 40 or above | Severe obesity |
People of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background have a higher risk of health problems at a lower BMI, so lower thresholds (23 and 27.5) are used. Waist size also matters: ideally your waist should be less than half your height.
How do these medicines work?
Semaglutide (Wegovy) mimics a gut hormone called GLP-1. Tirzepatide (Mounjaro) acts on both GLP-1 and GIP receptors. Both reduce appetite, help you feel full sooner and slow how quickly your stomach empties. They are given as a once-weekly injection, starting at a low dose that is increased gradually.
What does the evidence show?
- In the STEP 1 trial, adults taking semaglutide 2.4 mg lost an average of about 15% of their body weight over 68 weeks, compared with about 2.4% on placebo.
- In the SURMOUNT-1 trial, adults taking tirzepatide 15 mg lost an average of about 21% of their body weight over 72 weeks.
Results vary between people. Trials also show that much of the weight is often regained after stopping, which is why lasting changes to diet and activity are an essential part of treatment.
Who can use them?
On the NHS, NICE has approved semaglutide (TA875) and tirzepatide (TA1026) for adults who meet specific BMI criteria and have at least one weight-related health condition, with access being phased in. Privately, a prescriber will check your BMI, medical history and medicines before deciding whether treatment is safe and suitable for you. These medicines are not suitable for everyone, including people who are pregnant or planning pregnancy, or who have certain thyroid cancers or a history of pancreatitis.
Side effects and safety
- Common: nausea, diarrhoea, constipation, vomiting, indigestion and tiredness, usually easing over time. Eating smaller meals and stopping when full helps.
- Seek urgent help for severe, persistent stomach pain that may spread to your back (a possible sign of pancreatitis), or signs of gallbladder problems.
- Contraception: the MHRA advises that weight-loss injections should not be used during pregnancy. Tirzepatide may make oral contraceptives less effective in people who are overweight, so a barrier method or non-oral contraception is advised for four weeks after starting and after each dose increase.
- Dehydration: drink plenty of fluids, especially if you have sickness or diarrhoea.
Buy safely
The MHRA has warned about fake and unlicensed weight-loss pens sold online and through social media. Only obtain these medicines with a valid prescription, from a pharmacy you can verify on the General Pharmaceutical Council (GPhC) register. Never use a pen from an unregulated seller.
Getting the most from treatment
- Prioritise protein, vegetables and whole foods to preserve muscle while losing weight.
- Include strength exercises at least twice a week, as the UK activity guidelines recommend.
- Attend regular reviews so your progress, side effects and dose can be checked.
Sources and further reading
- Tirzepatide for managing overweight and obesity (TA1026) — NICE (https://www.nice.org.uk/guidance/ta1026)
- Semaglutide for managing overweight and obesity (TA875) — NICE (https://www.nice.org.uk/guidance/ta875)
- Obesity — NHS (https://www.nhs.uk/conditions/obesity/)
- Drug Safety Update — MHRA (https://www.gov.uk/drug-safety-update)
- Check the register — General Pharmaceutical Council (https://www.pharmacyregulation.org/registers)
Content reviewed on 26 October 2026 against current UK guidance. Guidance and eligibility can change; always check the linked sources for the latest information.
This healthcare guide is published for health literacy and educational purposes and does not replace personalized medical advice from a doctor or pharmacist. If you experience sudden, acute symptoms (such as crushing chest pain, signs of stroke, severe breathing difficulty, or an acute allergic reaction), call 999 immediately or visit your nearest Emergency Department. For non-urgent medical advice, call NHS 111 or consult our clinical pharmacists.
