- STEP 9 enrolled 407 adults with obesity and moderate knee osteoarthritis with at least moderate pain. Everyone got diet and activity counseling.1
- After 68 weeks, people on semaglutide lost 13.7% of their body weight on average, vs 3.2% on placebo.1
- Knee pain scores fell by 41.7 points with semaglutide and 27.5 points with placebo, on a 0 to 100 scale. Physical function also improved more with semaglutide.1
Why weight and knee pain are linked
Knee osteoarthritis is wear and change in the cartilage and bone of the knee joint. It causes pain, stiffness and trouble with walking, stairs and getting up from a chair. Extra body weight puts more load on the knees with every step. Earlier research had already shown that losing weight can ease osteoarthritis symptoms, including pain.1
What had not been well studied was whether a GLP-1 medicine could help people who already had painful knee osteoarthritis and obesity. STEP 9 was designed to answer that.
Who was in the trial
STEP 9 was a 68-week, double-blind, placebo-controlled trial run at 61 sites in 11 countries. Participants had obesity (a BMI of 30 or more) and a diagnosis of moderate knee osteoarthritis, confirmed both by a doctor's exam and by X-ray, with at least moderate pain.1
People were randomly assigned in a 2:1 ratio to once-weekly semaglutide 2.4 mg or placebo. Both groups also got counseling on a reduced-calorie diet and physical activity. At the start, the average knee pain score was 70.9 out of 100, which is a lot of pain.1
How pain was measured
The trial used the WOMAC pain score, a standard questionnaire for knee and hip osteoarthritis. It asks how much the knee hurts during everyday activities, such as walking on a flat surface or going up and down stairs. Here it was scored from 0 to 100, with higher numbers meaning more pain. Physical function was also measured with the SF-36 physical-function score, where higher numbers mean better function.1
What happened to weight
Weight loss was one of the two main goals of the trial. On average, the semaglutide group lost 13.7% of their starting weight, and the placebo group lost 3.2%. That is a gap of about 10 percentage points.1 If you want to compare this with the larger weight-loss trials, see what STEP 1 and SURMOUNT-1 found.
What happened to knee pain
Knee pain was the other main goal. Pain scores dropped by an average of 41.7 points with semaglutide and 27.5 points with placebo. The difference between the groups was statistically significant.1
What happened to physical function
| Semaglutide | Placebo | |
|---|---|---|
| Body weight, week 68 | −13.7% | −3.2% |
| WOMAC pain score (0–100, lower is better) | −41.7 points | −27.5 points |
| SF-36 physical function (0–100, higher is better) | +12.0 points | +6.5 points |
The SF-36 physical-function score was a key secondary goal. It rose by 12.0 points with semaglutide vs 6.5 points with placebo, again a significant difference. In plain terms, people on semaglutide reported a bigger improvement in everyday physical tasks.1
Side effects
Serious adverse events happened at similar rates in the two groups. More people stopped treatment for good because of side effects on semaglutide (6.7%) than on placebo (3.0%). Stomach and gut problems were the most common reason for stopping.1 For more on those, see GLP-1 nausea and other stomach side effects.
What STEP 9 does and does not tell us
- It studied a specific group. Adults with obesity and moderate knee osteoarthritis with at least moderate pain. Most were women, and the average BMI was over 40. The results may not apply to people with a lower BMI, arthritis in other joints, or severe end-stage disease.
- It does not separate weight loss from the medicine itself. The trial compared semaglutide with placebo. It was not built to show whether the pain relief came only from losing weight or partly from other effects of the drug.
- It did not test joint repair. The main results were about pain, function and weight. They do not show that cartilage grew back or that the disease reversed.
- It lasted 68 weeks. Longer-term effects on pain, and what happens if treatment stops, were not part of these results. Other trials show that weight often returns after stopping a GLP-1. See what happens when you stop.
- Novo Nordisk funded it. Novo Nordisk makes semaglutide and funded the trial, which is typical for drug trials of this size.1
The practical takeaway
For adults with obesity and painful knee osteoarthritis, STEP 9 shows that semaglutide, on top of diet and activity counseling, led to more weight loss, less knee pain and better physical function than counseling alone. The placebo group improved too, which is a reminder that support, movement and time also matter.
Whether a GLP-1 medicine fits your own situation, and how it would sit alongside physical therapy, exercise and other osteoarthritis care, is a question for your doctor or care team. Keeping a simple record of your weight and how your knees feel week to week can make that conversation more concrete. Keeping your strength up also helps the joints; see the protein research.
Sources
- Bliddal H, Bays H, Czernichow S, et al. Once-Weekly Semaglutide in Persons with Obesity and Knee Osteoarthritis (STEP 9). N Engl J Med. 2024;391:1573-1583. PubMed · doi:10.1056/NEJMoa2403664
