Weight Loss Injections or PRP for Knee Arthritis. What the Semaglutide Trial Means for Your Knee

Written By Sofian Youssef

With obesity and knee osteoarthritis, losing weight beats any joint injection. In the 407-patient STEP 9 trial in the New England Journal of Medicine semaglutide produced 13.7 percent weight loss against 3.2 percent on placebo, and knee pain fell 41.7 points against 27.5. That beats any PRP trial. PRP is for the knee still hurting once the weight has gone.

Key points

  • The STEP 9 trial, published in the New England Journal of Medicine in October 2024, randomised 407 adults with obesity and moderate knee osteoarthritis to weekly semaglutide 2.4 mg or placebo for 68 weeks, both with diet and exercise counselling. Mean weight change was minus 13.7 percent with semaglutide and minus 3.2 percent with placebo.

  • The WOMAC pain score in STEP 9 fell by 41.7 points from a baseline of 70.9 with semaglutide, against 27.5 points with placebo, and physical function on the SF-36 improved by 12.0 points against 6.5. Serious adverse events were similar in both groups, and 6.7 percent stopped for side effects against 3.0 percent.

  • The largest placebo-controlled PRP trial in the knee, 610 patients in Knee Surgery, Sports Traumatology, Arthroscopy in 2022, found PRP ahead of saline on WOMAC pain at 6, 12, 24 and 60 months. A 288-patient trial in JAMA in 2021 found no difference from placebo at 12 months.

  • Private semaglutide costs £120 to £300 a month in the UK depending on dose, and tirzepatide £150 to £280 or more. Specialist central London clinics charge from around £780 to £900 per image-guided PRP injection.

If you have knee arthritis and are carrying weight, message us on WhatsApp or email team@thewellnesslondon.com and a doctor will tell you which should come first, including if that means not injecting anything.

Why weight matters more to a knee than most injections

Every step puts several times body weight through the knee, and every kilogram lost removes several kilograms of load with every stride. That is the mechanical argument, and it has always been true. What the last 2 years added is a drug that produces the kind of weight loss diet alone rarely does, and a trial that measured what that did to arthritic knees. Fat tissue is also metabolically active and produces inflammatory signals that reach the joint, so weight loss reduces the chemical irritation of the joint as well as the load on it.

The STEP 9 trial is the first large randomised test of that idea using a modern weight loss drug. It enrolled 407 adults with a body mass index of 30 or more, a clinical and radiographic diagnosis of moderate knee osteoarthritis at Kellgren-Lawrence grade 2 or 3, and a WOMAC pain score of at least 40 out of 100. Most were women, the mean age was 56 and the mean BMI was 40.3. They were randomised 2 to 1 to weekly semaglutide 2.4 mg or placebo, both alongside counselling on physical activity and a reduced-calorie diet, for 68 weeks.

What STEP 9 found

The semaglutide group lost a mean of 13.7 percent of body weight. The placebo group, with the same diet and exercise counselling, lost 3.2 percent. On the WOMAC pain scale, where 0 is no pain and 100 is the worst, the semaglutide group fell 41.7 points from a baseline of 70.9. The placebo group fell 27.5 points. That is a difference of 14.1 points, and it was statistically significant. Physical function on the SF-36 improved by 12.0 points with semaglutide against 6.5 with placebo. Serious adverse events were similar in both groups, and 6.7 percent of the semaglutide group stopped because of side effects, mostly gastrointestinal, against 3.0 percent on placebo.

Two honest caveats belong beside those numbers. The trial was funded by the drug's manufacturer, and it did not include follow-up imaging, so it shows pain and function improving, not the joint changing. And the placebo group improved substantially too, by 27.5 points, which is a reminder that diet, exercise and attention on their own do a great deal for an arthritic knee. The drug added 14 points on top of that. That addition is larger than the difference PRP has shown over placebo in any knee trial.

Send us your knee X-ray or MRI report on WhatsApp and a doctor will tell you where your knee and your weight sit against the trial population.

How that compares with PRP

PRP has never produced a difference from placebo of that size in a knee trial. The largest placebo-controlled study, 610 patients in Knee Surgery, Sports Traumatology, Arthroscopy in 2022, found PRP ahead of saline on WOMAC pain, function and total score at 6, 12, 24 and 60 months, which is a durable result, but the margin was modest. The 288-patient trial in JAMA in 2021 found no difference from saline at 12 months. NICE describes the PRP evidence for knee osteoarthritis as limited in quality. A 2026 network meta-analysis of 21 randomised trials rated the certainty of the PRP evidence as low.

The 2 treatments are not doing the same thing. Semaglutide removes load and systemic inflammation from every joint in the body and improves blood pressure and blood sugar on the way. PRP delivers growth factors into one joint. For a patient with obesity and knee arthritis, the treatment that addresses the cause is the weight, and a doctor here would say so before drawing blood. Where PRP has a defensible place is after that, for the knee that still hurts once the weight is stable, or for the patient whose weight is not the problem.

Who should not be told to lose weight first

The weight argument applies to the patient in the trial, with a BMI of 30 or more, and it is worth being precise about who that is not. A patient of normal weight with knee arthritis gains nothing from semaglutide and should not be prescribed it for the knee. A patient who has already lost the weight and whose knee still hurts has removed the load and is now the patient the PRP trials describe. A patient with grade 4 bone-on-bone disease will get pain relief from weight loss, as the STEP 9 authors found benefit across baseline BMI, but the joint is still a surgical conversation.

There are also patients for whom the drugs are unsuitable, because of side effects, pancreatitis history, thyroid cancer history, pregnancy, or an eating disorder, and for whom weight loss has to be attempted the slower way. For them PRP remains a reasonable bridge treatment alongside the diet and exercise programme that the placebo arm of STEP 9 showed is worth 27 points on its own. The honest framing is that weight loss is the first treatment for the overweight arthritic knee, that the injections that produce it are a medical decision with a prescriber, and that PRP is a second step, not an alternative.

A 6 point check before choosing between them

Work through this before booking anything, including here.

  • What is your BMI. Above 30, weight loss is the treatment with the largest effect on knee pain in a trial, and it comes first.

  • What grade is the arthritis on X-ray. Grade 2 to 3 is the STEP 9 population and the PRP population. Grade 4 is a surgical conversation whatever else you do.

  • Have you done a structured diet and exercise programme. The placebo arm of STEP 9 gained 27.5 points on pain from that alone.

  • Is a weight loss injection safe for you. Pancreatitis, thyroid cancer history, pregnancy and eating disorders are reasons to discuss alternatives with a prescriber.

  • If you have already lost weight and the knee still hurts, that is the point at which PRP, with its 6-month peak and repeat courses, becomes the reasonable next step.

  • Is the clinic offering PRP asking about your weight. If not, it is treating the symptom and leaving the cause.

Our doctors put it this way. If you are carrying weight, the best injection for your knee goes into your abdomen once a week, not into your knee. PRP is for the knee that still hurts once that is done.

Message us on WhatsApp with your BMI if you know it, your X-ray findings, and what you have tried, and a doctor will give you a straight answer.

What neither treatment does

Neither semaglutide nor PRP regrows cartilage or reverses osteoarthritis. STEP 9 did not image the joints, so it shows pain and function improving, not structure. Semaglutide is not licensed for knee osteoarthritis, the trial was manufacturer funded, and 6.7 percent stopped for side effects. Its pain benefit depends on continuing the drug and keeping the weight off. PRP failed against placebo in a 288-patient trial, succeeded modestly in a 610-patient one, and is rated low certainty evidence. Neither is available on the NHS for knee arthritis, and neither replaces the diet and exercise programme that the placebo arm showed is worth 27 points on its own. Not everyone is suitable for PRP.

What each costs, and the arithmetic

Private semaglutide costs £120 to £300 a month in the UK depending on dose, and tirzepatide £150 to £280 or more, so a year of treatment runs from around £1,500 to £3,600 before consultation fees. Specialist central London clinics commonly charge from around £780 to £900 or more per image-guided PRP injection including assessment, with a course of 2 to 3 taking a plan past £2,000, and repeat courses at 6 to 12 months.

The reframing is what each pound buys. A year of semaglutide in an overweight patient buys the largest knee pain improvement measured in any trial, plus the blood pressure, blood sugar and cardiovascular effects that come with 13.7 percent weight loss. A PRP course in the same patient buys a modest, joint-specific improvement while the load and the inflammation continue. Spent in the right order, weight loss first, PRP if needed after, the same money treats the cause and then the symptom. Spent in the wrong order, it treats the symptom twice.

Care at The London PRP Clinic by The Wellness is doctor-led and priced below flagship central London rates, with your exact figure confirmed at the free suitability review, which for an overweight patient with knee arthritis often ends with a conversation about weight before any injection.

Ask about the cost for your knee on WhatsApp and we will explain what the review covers.

The free suitability review

Every enquiry here starts with a free, doctor-led suitability review rather than a booking. A GMC-registered doctor assesses your knee, your weight, your imaging and your medical history, and gives one of two answers. Either you are approved for PRP, with a clear plan and realistic expectations for a joint whose load has already been addressed, or you are not approved yet, with an honest explanation that weight loss is the first treatment, a referral to a prescriber where a weight loss injection is appropriate, and a date to reassess the knee.

We decline a meaningful share of the people who enquire. For an overweight knee, that share is high, because the treatment with the best evidence is not one we inject into the joint.

Why people choose The London PRP Clinic by The Wellness

  • An 87 percent patient success rate across treatments.

  • More than 187 five star reviews.

  • Every treatment performed by a GMC-registered doctor.

  • 100 supervised treatments completed by every clinician before they treat a patient here unsupervised.

  • A 32 percent average density increase reported in our hair restoration programme.

Frequently asked questions

Does Wegovy help knee arthritis

In the STEP 9 trial of 407 adults with obesity and moderate knee osteoarthritis, semaglutide 2.4 mg produced 13.7 percent weight loss and a 41.7 point fall in WOMAC pain against 27.5 on placebo over 68 weeks. It is not licensed for knee arthritis and works by reducing load and inflammation rather than treating the joint.

Is weight loss better than PRP for knee pain

For a patient with a BMI over 30, yes. The 14.1 point advantage of semaglutide over placebo in STEP 9 is larger than PRP has shown over placebo in any knee trial. PRP has a role afterwards, for the knee that still hurts once weight is stable, or for patients whose weight is not the problem.

Can I have PRP while on Mounjaro or Wegovy

Yes, there is no known interaction, but the order matters. A doctor would usually let the weight loss take effect first, since the knee may improve enough that PRP is unnecessary, then reassess at 6 to 12 months and offer PRP for residual pain.

Does Mounjaro help knee osteoarthritis

Tirzepatide has not been tested in a dedicated knee osteoarthritis trial, but it produces greater weight loss than semaglutide in head-to-head trials, and the STEP 9 authors found pain benefit tracked weight loss. It is reasonable to expect a similar or larger effect, though it has not been measured.

How much weight do I need to lose for knee pain

STEP 9 measured 13.7 percent, which for a 100 kg patient is around 14 kg, and found benefit across baseline BMI. The placebo group lost 3.2 percent and still gained 27.5 points on pain from diet and exercise, so every kilogram counts and the programme matters as much as the number.

Should I lose weight before a knee replacement

Usually. Surgeons often ask for weight reduction before listing, since it lowers surgical risk and improves recovery, and a weight loss injection can achieve in a year what diet alone rarely does. It may also reduce pain enough to defer the operation. The decision belongs with your surgeon.

References

This article is for general information and does not replace a medical assessment. Weight loss injections are prescription medicines with contraindications and side effects. Speak to one of our doctors, and a prescriber, before deciding on any treatment.

Sofian Youssef
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