Semaglutide and Knee Osteoarthritis: The Number Behind the Headline

Knee osteoarthritis and obesity travel together, and losing weight has always helped. What was not known is whether these drugs help specifically, and a 68-week randomized trial has now tested it: pain scores fell 41.7 points on semaglutide.^[1] The number worth carrying is smaller. Placebo fell 27.5 points, so the drug’s own contribution is the 14.2-point gap — real, statistically solid, and about a third of the figure you will see quoted.

What the trial did

STEP 9 enrolled 407 people across 61 sites in 11 countries, all with obesity and a clinical and radiologic diagnosis of moderate knee osteoarthritis, all in at least moderate pain. Randomization ran two-to-one, semaglutide 2.4 mg against placebo, across 68 weeks — and both arms also got counseling on activity plus a reduced-calorie diet.^[1]

Pain was a co-primary endpoint alongside weight, measured on the WOMAC index — the standard instrument in osteoarthritis research, scored 0 to 100 with higher meaning worse. Participants started at a mean of 70.9, which is substantial pain, with a mean BMI of 40.3. 81.6% were women.

Why the placebo column matters so much here

Osteoarthritis trials are notorious for large placebo responses, and this one is a good example: people given a placebo injection and lifestyle counseling improved their pain score by 27.5 points, roughly two thirds of the improvement seen on the drug.

Two thirds of the pain improvement happened without the drug.

That is not a reason to dismiss the result. The gap between the arms is what a randomized trial exists to measure, it was statistically significant, and 14.2 points on WOMAC is a difference people notice. It is a reason to be suspicious of any source quoting 41.7 without the 27.5 beside it, because that framing credits the drug with the trial’s entire effect.

Function, not just pain

Physical function improved more on the drug as well: 12.0 points against 6.5 on the SF-36 physical-function scale.^[1] That matters because pain and function come apart in arthritis — people can report less pain while still avoiding stairs — and a treatment that moves both is doing something more useful than one that only moves the score.

Here the ratio is roughly two to one rather than three to two, which is the more favorable comparison of the two for the drug.

What it cost

Serious adverse events were similar between the groups. Discontinuation because of side effects was not: 6.7% stopped on semaglutide against 3.0% on placebo, with gastrointestinal problems the commonest reason.^[1] That is the familiar trade of this class, and our side-effect timeline and constipation guide cover what it looks like week by week.

One thing the trial cannot tell you: whether any of this changes the joint itself, or the eventual need for a replacement. Sixty-eight weeks measures symptoms. Osteoarthritis is a structural disease that unfolds over decades, and no trial has followed these drugs long enough to say anything about that. The closest evidence is observational: a 2026 database study found that people with knee osteoarthritis who took semaglutide or tirzepatide for three years had fewer knee replacements over eight years (hazard ratio 0.72), and its authors say trials are needed to establish cause.^[2] For back and joint pain beyond the knee, what the trials counted is covered separately.