Most people receiving a Zilretta injection begin noticing pain relief within the first few days, but the greatest improvement typically occurs around week 7 after the injection. Unlike standard steroid shots that deliver their full dose almost immediately, Zilretta uses a slow-release technology that gradually releases medication into the knee joint over roughly 12 weeks.
What to Expect in the First Week
Zilretta contains the same active steroid found in many conventional knee injections, but it’s embedded in tiny biodegradable microspheres about the width of a fine hair. Shortly after injection, a small burst of medication releases from the surface of these microspheres within the first hours to days. This initial release is why some people feel a degree of relief relatively quickly, though it’s typically modest compared to what comes later.
During this early window, you may also notice some soreness or swelling at the injection site. Injection site reactions are the most common side effect, occurring in 1 to 10 percent of patients. Headache and joint pain have also been reported. These effects generally resolve on their own.
Peak Relief Around Week 7
Clinical trial data submitted to the FDA showed that the largest improvement in pain, and the biggest gap between Zilretta and placebo, appeared around week 7. This is noticeably later than a traditional cortisone shot, which often peaks within the first week or two and then fades. Zilretta’s timeline is slower because the microspheres need time to break down and release their full payload of medication.
After that initial burst phase, water gradually penetrates the polymer shell of each microsphere, creating tiny channels that allow the steroid to diffuse outward steadily. This sustained-release phase is responsible for the prolonged, building relief over several weeks. Eventually, the microspheres fragment entirely and release whatever medication remains. The entire process spans about 12 weeks from injection to full breakdown.
So if you’re a week or two in and feel underwhelmed, that’s expected. The injection is designed to ramp up rather than hit hard and fade.
How Long the Relief Lasts
Because the microspheres release medication over approximately 12 weeks, most patients experience meaningful pain relief for about three months. Some people report benefits that extend slightly beyond that window as residual inflammation stays controlled, but the active drug delivery is essentially complete by the 12-week mark.
One important detail: the manufacturer has not established the safety or efficacy of repeat injections. The clinical trials primarily studied a single injection, so there isn’t robust data guiding how soon you could receive another one in the same knee. If your pain returns after the effects wear off, your doctor will need to weigh options on a case-by-case basis.
How Zilretta Differs From Standard Cortisone Shots
A conventional cortisone injection delivers its entire dose of steroid into the joint at once. That means fast relief, often within 24 to 48 hours, but the drug clears the joint relatively quickly too. Pain relief from a standard shot commonly lasts four to six weeks, sometimes less. The steroid can also spill into the bloodstream more readily, which is why traditional injections sometimes cause a temporary spike in blood sugar or other systemic effects.
Zilretta’s microsphere design keeps more of the medication inside the joint for longer. The polymer breaks down into lactic acid and glycolic acid, both of which the body processes naturally. This slower, localized delivery is the reason the onset feels more gradual but the duration stretches to roughly three months. In clinical trials, Zilretta also produced smaller blood sugar elevations in diabetic patients compared to standard steroid injections, which can matter if you’re managing type 2 diabetes alongside knee osteoarthritis.
Who Gets Zilretta
Zilretta is FDA-approved specifically for osteoarthritis pain in the knee. The prescribing information does not restrict it to a particular severity level, so it can be considered whether your arthritis is mild, moderate, or severe. In practice, doctors often recommend it for patients who haven’t gotten adequate or lasting relief from standard cortisone injections, or for those who want longer-duration relief without more frequent shots.
It is not approved for other joints such as the hip, shoulder, or ankle. And because it’s a corticosteroid, the same general cautions apply: it’s not a cure for osteoarthritis, it won’t regenerate cartilage, and it works by reducing inflammation and the pain that comes with it. The goal is functional improvement, making it easier to walk, climb stairs, and participate in physical therapy during the months of active relief.
Making the Most of the Relief Window
Since Zilretta’s effects build over several weeks and last about three months, the relief window is a practical opportunity to strengthen the muscles around your knee. Physical therapy or a structured exercise program during this period can improve joint stability and potentially extend the functional benefit beyond the injection itself. Low-impact activities like cycling, swimming, and walking are commonly recommended alongside the injection.
Keeping realistic expectations about timing helps too. If you’re getting the injection before a vacation or a specific event, plan for the fact that you’ll feel progressively better over weeks, not overnight. The strongest relief is likely to arrive around the six- to eight-week mark, with continued benefit through week 12.