Hyaluronic acid injections are used for two main purposes: cosmetic facial fillers and joint lubrication for knee osteoarthritis. Both carry side effects, though the type and severity differ. The most common reactions are mild and short-lived, particularly swelling at the injection site, which affects roughly 41% of people receiving facial fillers. Serious complications are rare but worth understanding before you go in.
Common Side Effects of Facial Fillers
A systematic review of facial filler studies found swelling to be the single most frequent reaction, occurring in about 41% of patients. After that, the rates drop considerably: bruising affects around 11%, pain about 10%, and lumps or bumps about 9%. Redness at the injection site occurs in roughly 5% of cases. Less common reactions include tenderness (about 2%), firmness (about 2%), and itching (under 1%).
These reactions are typically mild and resolve on their own within a few days. Bruising and swelling are especially common around the lips and under-eye area, where the skin is thinner and blood vessels sit closer to the surface. If you’re taking blood-thinning medications or supplements like aspirin, ibuprofen, or fish oil, bruising tends to be more pronounced. Stopping unnecessary blood thinners before your appointment can reduce this risk.
Common Side Effects of Knee Injections
When hyaluronic acid is injected into a knee joint for osteoarthritis, the most common side effect is a local reaction at the injection site: mild pain, warmth, or swelling that typically resolves within one to three days. Higher-molecular-weight formulations tend to cause more of these flare-ups than lower-weight versions.
A pooled analysis of six trials involving over 800 patients found that the risk of a local flare reaction was not significantly higher with hyaluronic acid injections compared to control injections. In other words, much of the discomfort people experience may simply come from having a needle in the joint rather than from the hyaluronic acid itself.
A more intense reaction called “pseudosepsis” can occur, though it’s uncommon. This involves marked joint inflammation with significant swelling and pain, usually appearing within 24 to 72 hours after the injection. It mimics a joint infection but isn’t one. The distinction matters because a true infection requires urgent treatment, so sudden severe joint swelling after an injection should always be evaluated promptly.
The Tyndall Effect: A Visible Cosmetic Issue
When facial filler is placed too close to the skin’s surface or injected in large amounts in one spot, it can create a bluish discoloration visible through the skin. This is called the Tyndall effect, and it’s distinct from a bruise. While a bruise fades over days, this bluish tint persists because the filler itself is refracting light beneath the skin. The area often looks slightly raised or lumpy as well.
If caught immediately, firm massage can sometimes flatten and disperse the filler. The longer you wait, the less likely massage is to work. Beyond the first few days, the main treatment option is an enzyme called hyaluronidase, which dissolves hyaluronic acid. In minor cases, a small puncture with a needle to express the filler can resolve the issue. Some people simply opt to cover the discoloration with makeup if they’d rather avoid additional procedures.
Delayed Nodules and Granulomas
Weeks, months, or occasionally years after a filler injection, some people develop firm lumps under the skin. These delayed-onset nodules can be inflammatory (red, tender, sometimes associated with infection) or non-inflammatory (painless bumps from filler that has shifted or clumped). A four-year retrospective study found that hypersensitivity reactions, including nodules, occurred in 0.6 to 0.8% of patients receiving hyaluronic acid fillers.
True foreign body granulomas, where the immune system walls off the filler material in a persistent inflammatory reaction, are rarer still, with an estimated incidence between 0.01 and 1%. Treatment depends on the type: non-inflamed nodules can often be dissolved with small amounts of hyaluronidase, while inflammatory nodules may require a course of oral antibiotics first, followed by enzyme injections if they don’t resolve.
Vascular Occlusion: The Most Serious Risk
The most dangerous complication of facial filler injections is vascular occlusion, where filler material blocks a blood vessel and cuts off blood supply to the surrounding tissue. This is rare, but it can cause permanent scarring or, in the worst cases, vision loss if filler reaches the blood vessels supplying the eye.
The warning signs follow a recognizable pattern. First, you’ll notice sudden, escalating pain during or shortly after the injection, either at the injection site or in a nearby area. The skin may turn pale or white as blood flow is interrupted. Within minutes to hours, this pallor gives way to a purple, mottled, net-like pattern as deoxygenated blood builds up in the tissue. Over the following days, if untreated, the tissue darkens further as it begins to die.
The typical timeline runs in stages: immediate pallor (seconds to minutes), purple mottling (within 24 to 36 hours), pustule formation (around 72 hours), and eventually dead tissue forming a dark, hardened scab over the following days. Ischemic changes usually appear instantly or within a few hours of injection, though in rare cases they can show up days later.
This is a medical emergency. Treatment involves high doses of hyaluronidase to dissolve the filler blocking the vessel. Protocols call for 450 to 1,500 units delivered in multiple rounds, far more than the small doses used for cosmetic corrections. When ultrasound guidance is available to pinpoint the exact location of the blockage, much lower doses (35 to 50 units) can be effective. The speed of treatment directly determines the outcome: the sooner blood flow is restored, the less tissue damage occurs.
Who Faces Higher Risk
Active infections near the injection site are a clear reason to postpone treatment. This includes dental infections, cold sores, or any skin infection in the area being treated. If you have a history of cold sores and are getting lip filler, a flare-up can be triggered by the injection itself, so preventive antiviral medication is worth discussing beforehand.
Allergies to hyaluronic acid are uncommon but do exist. Some filler syringes contain lidocaine (a numbing agent), which is a separate allergy to be aware of. One case of an angioedema-type swelling reaction has been reported following lip injection with hyaluronic acid filler.
Autoimmune conditions like lupus, rheumatoid arthritis, or scleroderma do not appear to increase complication risk based on current evidence. Immunosuppression also hasn’t been shown to raise the risk of hyaluronic acid filler complications specifically. These are common concerns, but no causal relationship has been established between filler use and autoimmune flare-ups.
What Normal Recovery Looks Like
For facial fillers, expect some combination of swelling, tenderness, and possibly bruising for the first two to five days. Swelling is most noticeable in the first 24 to 48 hours and can make the results look exaggerated before settling. Ice packs and keeping your head elevated help. Bruising, when it occurs, typically follows the same timeline as any bruise: shifting from purple to yellow-green over about a week.
For knee injections, mild soreness and stiffness at the injection site for one to three days is standard. Applying ice and avoiding intense activity for the first 24 to 48 hours helps the joint settle. If swelling worsens rather than improves after the first couple of days, or if the joint becomes hot and red, that warrants a call to your provider to rule out infection or pseudosepsis.