Zilretta is not a gel injection. It is an extended-release injectable suspension, meaning it consists of tiny solid microspheres mixed with a liquid diluent right before injection. The confusion likely comes from the fact that Zilretta uses a unique delivery system that works differently from a standard steroid shot, but the physical form is a powder-and-liquid suspension, not a gel.
What Zilretta Actually Is
Zilretta is a corticosteroid (triamcinolone acetonide) packaged inside microscopic polymer beads called microspheres. These microspheres are made from a biodegradable material called PLGA, the same type of polymer used in dissolving surgical sutures. The product comes as a kit with two vials: one containing the dry microsphere powder and the other containing 5 mL of liquid diluent. A healthcare provider mixes them together immediately before injection, creating a milky suspension that is then injected into the knee joint.
The final product looks and behaves like a liquid, not a gel. It flows through a standard needle and delivers 32 mg of the steroid in a single 5 mL injection directly into the knee.
How the Microspheres Work
In a traditional cortisone shot, the steroid dissolves quickly in the joint fluid. That gives fast relief, but the drug also clears out of the joint rapidly, often within days. Zilretta’s microspheres slow that process down considerably. The steroid is embedded in crystalline form inside each tiny bead. Once injected, water from the joint fluid gradually seeps into the microspheres through small channels, slowly dissolving and releasing the medication over weeks rather than days.
This extended-release design keeps the drug level in the joint more consistent and lower at any given moment compared to a standard injection. That steady, controlled release is the main reason Zilretta exists: it aims to provide longer-lasting pain relief while reducing the spike of steroid that floods the joint (and sometimes the bloodstream) right after a conventional shot.
What to Expect After the Injection
Most people begin to notice improvement within the first week. In clinical trials, patients receiving Zilretta reported pain scores about two points lower on a standard 11-point scale by week one, compared to about one point of improvement in those who received a placebo. Because the drug releases gradually, the relief tends to build and hold over the following weeks rather than peaking immediately and fading.
Zilretta is approved only for osteoarthritis pain in the knee. It is given as a single injection, and current labeling notes that the safety and effectiveness of repeat injections have not been established. That does not necessarily mean a second injection is dangerous, but there is not yet enough clinical data to support routine repeat dosing.
Common Side Effects
Because Zilretta delivers a corticosteroid, its side effects overlap with those of any steroid injection. The most commonly reported include skin changes such as acne, dryness, or bruising near the injection site. Some people experience nausea, bloating, appetite changes, headache, or trouble sleeping. Increased sweating, slower wound healing, and changes in menstrual periods have also been reported. These effects are generally mild and temporary, reflecting the steroid gradually entering the bloodstream as the microspheres break down.
How Zilretta Differs From Other Knee Injections
The knee injection landscape includes several options that can be confusing to compare. Hyaluronic acid injections (sometimes called “gel shots” or viscosupplementation) are thick, gel-like substances meant to lubricate the joint. That gel-like quality is probably why some people wonder whether Zilretta is also a gel. It is not. Zilretta is a steroid, not a lubricant, and its physical form is a suspension of solid particles in liquid rather than a viscous gel.
Compared to a standard cortisone shot, Zilretta contains the same active ingredient (triamcinolone acetonide) but packages it differently. A conventional injection delivers the full dose immediately. Zilretta meters it out over time through its microsphere technology. Both are injected the same way, into the joint space of the knee, and neither feels like a gel going in.