The short version: in a large trial of people with obesity, a new investigational drug called retatrutide eased knee arthritis pain. That is encouraging for anyone who wants to keep walking. It is not evidence that cartilage grows back, and the drug is not yet something you can be prescribed in the UK. [1] [2]

I am Professor Paul Lee, and the people I meet from Lincolnshire and the wider East Midlands usually ask the same thing about their knees: how do I keep going? They want the garden, the dog walk, the stairs and the grandchildren, without pain deciding the day for them.

So when a major trial reports less knee pain in people taking a weight-loss medicine, I read it with one question in mind. Does it help people stay on their feet?

What the trial found

Within TRIUMPH-1, the knee osteoarthritis subgroup was made up of people living with obesity. Over 80 weeks, those given retatrutide reported clearly better WOMAC knee pain scores than those given placebo. WOMAC is a standard questionnaire that asks about arthritis symptoms such as pain. [1]

That is a real result, and I do not want to undersell it. But it describes a specific group. It tells us nothing certain about a person of healthy weight with a meniscus tear or a small area of damaged cartilage, and an average across many people cannot promise what will happen to you.

What it does not show

Less pain is not the same as a repaired joint. The trial does not establish cartilage regeneration, and it does not show that arthritis is reversed or that knee replacement is avoided. [1]

Retatrutide is also still investigational. It is not authorised by the MHRA for use in the UK. [2] Not everyone with a sore knee should be looking for it.

I work in regenerative medicine, so I am strict about this language. Pain relief matters in its own right. We should simply call it what it is.

Why strong legs are part of this story

Think about what your thigh muscles do all day. The quadriceps take load off the joint, steady the knee and let you rise from a chair, walk on uneven ground and climb steps.

When someone loses a lot of weight, I do not want them to end up as a lighter version of the same patient with weaker legs. Shedding excess fat may ease the load on the knee, but I do not want useful muscle disappearing alongside it.

The evidence here is mixed, and I think honesty serves you better than enthusiasm. A phase 2 body-composition substudy, run in volunteers who have type 2 diabetes, recorded lean tissue loss as well as fat loss, in a proportion the authors describe as similar to other obesity treatments. Lean tissue is not the same thing as skeletal muscle, and a scan of body composition cannot tell us whether strength changed. So nobody should assume that muscle is protected automatically. [4]

It is why I believe any medical weight loss for the knee should come with a check on muscle and movement. Sensible nutrition and suitable exercise stay important while weight is being lost under medical supervision. [2]

Why arthritis is not simply a worn-out joint

I have never liked the label of wear and tear for arthritis. The knee is a living part of the body. Pain and function depend on cartilage and bone, but also on the synovium (the lining of the joint), the muscles around it, the forces passing through it, inflammation and general metabolic health.

Excess body weight shows this well. More mass means more load through the knee, yet the link with arthritis is not only mechanical. Body fat is not inert: it is active tissue, and chemical and inflammatory signals from it are increasingly thought to matter in osteoarthritis.

That suggests a broader goal than just dropping pounds. The aim is to improve metabolic health, reduce excess fat, keep or build muscle, move well and treat the joint itself when that is needed.

How retatrutide works, in plain terms

Retatrutide acts on three hormone receptors at once: GLP-1, GIP and glucagon. One medicine, three metabolic signals. [3]

Three hormones, briefly

GLP-1 helps to control appetite and encourages insulin release when blood glucose is high. GIP also supports insulin release that depends on glucose. Retatrutide pairs these with activity at the glucagon receptor. [3]

It helps to keep glucagon and insulin apart in your mind, because they do different jobs.

Insulin lets the body put glucose to work or store it. Blood glucose falls because glucose is drawn out of the bloodstream into tissues such as muscle, and because the liver produces less.

Glucagon keeps blood glucose steady between meals. It tells the liver to release glucose, and it also affects how the body handles fat and amino acids.

So retatrutide does not behave like insulin. Instead it stimulates the GLP-1, GIP and glucagon receptors at the same moment, giving a blended metabolic effect.

Why glucagon draws interest

The glucagon part is of interest because it may add to energy expenditure, meaning the energy the body burns, and to how energy fuels are handled. Early experiments in mice showed a glucagon-linked rise in energy expenditure. That explains the thinking behind the drug, but it does not tell us how much this contributes to lasting weight loss in people. [3]

I would also steer clear of saying glucagon switches on muscle. We have no human evidence that retatrutide activates or strengthens skeletal muscle directly.

A possible future for knee care

Could treatments like this one become part of looking after knee arthritis? I think they could. To be clear, that is my reading of where the field may go, not something already proven. Retatrutide is not an approved treatment for knee osteoarthritis, and the current trials do not demonstrate cartilage regeneration or any change to the underlying disease.

For selected people with knee arthritis who also have metabolic disease or excess weight, care might one day treat the joint and its metabolic surroundings together. That could include:

• good nutrition and, where right for you, medically supervised weight management;
• protecting muscle through progressive strengthening;
• work on how you walk, squat and rise from a chair; and
• specific treatment aimed at the knee itself.

If that day comes, I would want us to measure more than a pain score and the bathroom scales. I would want to follow:

• muscle strength and what you can do day to day;
• walking, squatting and sit-to-stand ability;
• knee symptoms over time;
• body composition and metabolic health; and
• scans of the joint where they are appropriate.

The test of success should be whether you have become more capable, not merely lighter.

I use the phrase metabolic joint preservation only to describe that aim: weighing metabolic health together with symptoms, strength and the joint. Nothing here suggests that retatrutide protects the structure of cartilage.

Can we improve the health of the whole person so that the knee has a better future?

What to do while the research continues

Please do not put your knees on hold for a drug that is still under investigation. A painful knee deserves assessment with the options available now. At Lincolnshire Knee Clinic we assess knee osteoarthritis with those options, looking at your symptoms, your strength, how you move and what you want to be doing.

Whatever treatment is right, I would want your legs strong and your knee moving. The research is interesting. Keeping you active is the goal.

References and further reading

Note that the TRIUMPH-1 source is a conference presentation from the manufacturer. The mechanism paper and the body-composition paper answer separate questions.

1. Lilly: TRIUMPH-1 Phase 3 results, ADA Scientific Sessions, June 2026 (conference presentation).
2. MHRA: No summer shortcut for safe weight loss, 24 July 2026.
3. Coskun et al. LY3437943: from discovery to clinical proof of concept. Cell Metabolism, 2022.
4. Coskun et al. Effects of retatrutide on body composition in people with type 2 diabetes: a substudy of a phase 2 randomised trial. Lancet Diabetes & Endocrinology, 2025.