Quick answer: Weight-loss injections such as Wegovy (semaglutide) and Mounjaro (tirzepatide) are licensed for weight management, not for arthritis. Losing weight can reduce knee and hip pain in osteoarthritis, and a large trial found semaglutide reduced knee pain in people with obesity and knee osteoarthritis. They are available on the NHS only for people who meet set criteria. Side effects are common, and protecting muscle strength matters.
What these medicines are
Wegovy contains semaglutide and Mounjaro contains tirzepatide. Both are weekly injections that copy natural gut hormones involved in appetite and blood sugar control. They make you feel fuller for longer and reduce hunger, which helps people eat less and lose weight. Similar medicines, under different brand names, are used to treat type 2 diabetes.
In the UK they are licensed for weight management in adults who meet certain criteria, alongside a reduced-calorie diet and increased physical activity. They are not licensed as a treatment for arthritis itself.
How weight loss can help arthritis
Extra body weight increases the load on weight-bearing joints such as the knees and hips. Fat tissue also releases substances that promote inflammation. For people with osteoarthritis who are carrying extra weight, NICE recommends weight loss as part of core treatment, alongside exercise and education, because even modest weight loss can reduce pain and improve function.
There is growing research into weight-loss medicines specifically in people with arthritis. One large trial, published in 2024, found that people with obesity and knee osteoarthritis who took semaglutide lost more weight and reported less knee pain and better physical function than those given a placebo. Research is ongoing, including into whether these medicines have effects on joints beyond weight loss. For people with psoriatic arthritis and other inflammatory conditions, weight loss may help treatment work better, but more research is needed.
Who can get them on the NHS?
NICE has approved both medicines for use on the NHS for some adults living with obesity, usually with at least one weight-related health condition, and with specific body mass index (BMI) thresholds. Access has been phased, with priority given to people with the greatest clinical need, and the arrangements differ between specialist weight management services and GP-led care and across the four UK nations. Eligibility is based on weight and health conditions, not on having arthritis alone.
If you are interested, ask your GP whether you might be eligible and what local weight management support is available. Your GP can also discuss other options, such as NHS weight management programmes and the NHS Digital Weight Management Programme for people with diabetes or high blood pressure.
Buying privately: staying safe
Many people buy these medicines privately through online pharmacies. If you do:
- Only use a UK-registered pharmacy. You can check registration with the General Pharmaceutical Council.
- Expect a proper health assessment, including your weight, medical history and current medicines. Be honest about your arthritis and the medicines you take.
- Never buy from social media, beauty salons or unregulated websites. The MHRA has warned about fake and unsafe weight-loss pens.
- Tell your GP and rheumatology team that you are taking one, so it is on your record.
Side effects
Side effects are common, especially when starting or increasing the dose, and often improve with time. They include:
- Feeling sick, vomiting, diarrhoea or constipation.
- Indigestion, bloating and burping.
- Tiredness and dizziness.
- Hair thinning, sometimes linked to rapid weight loss.
Less common but serious side effects include inflammation of the pancreas (severe tummy pain spreading to the back, which needs urgent medical help) and gallbladder problems. Dehydration from vomiting or diarrhoea can affect the kidneys, which matters more if you also take anti-inflammatory painkillers. Read the patient leaflet and seek medical advice if you are worried.
Protecting muscle and bone
When people lose weight quickly, some of the weight lost is muscle. Muscle strength is vital for supporting arthritic joints, balance and staying independent, particularly as you get older. To protect it:
- Do strength exercises at least twice a week. See our guide to resistance training for arthritis.
- Eat enough protein at each meal, even when your appetite is low.
- Keep active every day, for example with walking or swimming.
- Ask your GP about bone health if you have other risk factors for osteoporosis.
Using them alongside arthritis medicines
- There are no well-known major interactions with common DMARDs or biologics, but always check with your pharmacist or prescriber.
- These medicines slow stomach emptying, which may affect how some tablets are absorbed. Your pharmacist can advise.
- Mounjaro can make oral contraceptive pills less effective, particularly when starting or increasing the dose, so extra contraception may be needed. These medicines should not be used in pregnancy. This matters if you take arthritis medicines that also need careful pregnancy planning, such as methotrexate or leflunomide.
- If you become unwell with vomiting or diarrhoea, ask your pharmacist or GP whether any medicines, such as NSAIDs, should be paused while you recover. Do not change doses of your arthritis medicines without advice.
- Tell your surgical team and anaesthetist if you take one before any operation, including joint surgery.
What happens when you stop?
Research shows that many people regain weight after stopping these medicines. Building lasting habits around food, activity and sleep while taking them gives you the best chance of keeping benefits. The medicines work best as one part of a wider plan, not on their own.
Weight-loss support without injections
Injections are not suitable or available for everyone. Many people with arthritis lose weight through changes to diet and activity, with support from their GP, a dietitian, a weight management programme or a physiotherapist. See our diet guide and exercise guide.
Frequently asked questions
Can Mounjaro or Wegovy help knee arthritis?
They are not licensed for arthritis, but weight loss can reduce knee pain in osteoarthritis. A large trial found semaglutide reduced knee pain in people with obesity and knee osteoarthritis.
Can I get weight-loss injections on the NHS for arthritis?
Eligibility is based on your BMI and weight-related health conditions, not arthritis alone. Ask your GP whether you might qualify.
Can I take Mounjaro with methotrexate?
There is no well-known major interaction, but check with your pharmacist and tell your rheumatology team. Both need careful pregnancy planning.
Will I lose muscle on weight-loss injections?
Some muscle loss is common with rapid weight loss. Strength exercise and enough protein help protect muscle.
Are weight-loss injections safe to buy online?
Only buy from a UK-registered pharmacy that carries out a proper health assessment. Never buy from social media or unregulated sellers.
Do I need to stop weight-loss injections before joint surgery?
Tell your surgical team and anaesthetist. They will advise what to do, as these medicines slow stomach emptying.
Sources and further reading
- NICE TA875: Semaglutide for managing overweight and obesity
- NICE TA1026: Tirzepatide for managing overweight and obesity
- NICE NG226: Osteoarthritis in over 16s
- NHS: Obesity treatment
This guide is for general education and is not a substitute for personal medical advice. Everyone's situation is different, so always follow the advice of your own GP, pharmacist or rheumatology team. If you feel very unwell, call 999 or go to A&E. For urgent advice, contact NHS 111.