Most people notice initial pain relief from Zilretta within about 4 days of the injection. In clinical trials, the median time to a meaningful reduction in pain (at least 30% improvement) was 4 days, which is roughly the same as a standard steroid injection. The real difference with Zilretta isn’t how fast it kicks in, but how long the relief lasts.
What to Expect in the First Week
Zilretta contains the same active steroid used in conventional knee injections, triamcinolone acetonide, but it’s embedded in tiny biodegradable microspheres that dissolve slowly inside the joint. Some of the medication releases right away, which is why you can feel improvement within a few days. The rest releases gradually over the following weeks.
Don’t be surprised if the first few days feel underwhelming compared to stories you’ve heard about standard cortisone shots. A conventional injection dumps its entire dose at once, so some patients feel dramatic relief within 24 to 48 hours. Zilretta’s initial relief builds more steadily because the drug is metering itself out over time.
When Pain Relief Peaks
Clinical trial data shows Zilretta reduced daily pain scores by 50% or more starting at week 2, and that level of relief held through week 16. By comparison, patients who received a standard steroid injection only hit that 50% threshold at week 6 before their relief faded. The strongest separation between Zilretta and a conventional injection appeared between weeks 5 and 19, where Zilretta consistently outperformed the standard shot on pain scores every single week.
At the 12-week mark in one trial, about 28% of Zilretta patients reported zero knee pain, compared to roughly 8% of those who received a conventional steroid injection. By week 16, about 20% of Zilretta patients were still pain-free.
How Long the Relief Lasts
Zilretta was designed to provide at least 12 weeks of meaningful pain relief, and the clinical data supports that. In patients with moderate to severe knee osteoarthritis pain, the treatment effect persisted for 16 weeks or longer when compared to placebo. By week 24 (about 6 months), the difference between Zilretta and placebo was no longer detectable in trials.
So the realistic window is roughly 3 to 5 months of noticeable benefit for most people, with a gradual tapering rather than a sudden drop-off. Your individual experience will depend on factors like how severe your osteoarthritis is, your activity level, and your body weight.
How Zilretta Compares to a Standard Steroid Shot
Both injections start working around the same time. The key difference is what happens after that first week. A standard cortisone injection releases its full dose quickly, which means the steroid floods the joint and then clears out. Pain relief from a conventional shot typically fades within 4 to 6 weeks for most people.
Zilretta’s microsphere technology slows that process down considerably. Less of the steroid escapes into the bloodstream compared to a standard injection, which means more of it stays in the knee joint where it’s needed. This is why Zilretta showed better scores for pain, stiffness, and physical function at the 4, 8, and 12-week marks compared to the conventional version.
Getting a Second Injection
If the first injection helped but the pain eventually returns, a repeat injection is an option. In a Phase 3 study, patients were eligible for a second dose starting at week 12, though the median time to a second injection was about 16.6 weeks (just under 4 months), with a range of roughly 11 to 27 weeks depending on when patients felt they needed it again.
The repeat injection was well tolerated. Side effect rates were actually slightly lower after the second injection (35%) than after the first (41%). Importantly, X-rays taken up to a year after treatment showed no signs of joint damage, bone changes, or cartilage breakdown, which are concerns that sometimes come up with repeated steroid use in a joint.
Who Zilretta Is For
Zilretta is FDA-approved specifically for osteoarthritis pain of the knee. It’s been studied in patients with moderate to severe pain, so it’s typically considered when knee OA is significantly affecting your daily life or when other treatments like oral pain relievers, physical therapy, or standard injections haven’t provided enough relief. It is not approved for other joints like hips or shoulders, and it’s a single-joint injection, meaning each knee needs its own dose if both are affected.