What Are Intravitreal Injections and How Do They Work?

Intravitreal injection is a procedure in which medication is delivered directly into the vitreous, the gel-like substance filling the back of the eye. It has become one of the fastest-growing medical procedures in the world, climbing from roughly 2,400 Medicare claims in 1993 to an estimated 7 million injections in the United States alone by 2016, with over 20 million performed worldwide that same year.¹ That growth reflects how effectively the technique treats conditions that once had no good options, from age-related macular degeneration to diabetic eye disease. But a needle in the eye is not a casual thing, and what patients and their families want to know goes well beyond the textbook definition.

Why Inject Directly Into the Eye

The eye is remarkably good at keeping foreign substances out. A set of biological barriers, collectively called the blood-ocular barriers, prevent most drugs circulating in the bloodstream from reaching the retina in useful concentrations. Eye drops fare little better for diseases of the back of the eye; most of the drug is washed away by tears or absorbed into nearby blood vessels long before it can diffuse to the retina. Intravitreal injection sidesteps all of that by placing the drug right where it needs to be, achieving high concentrations in the vitreous while avoiding the side effects that come with giving the same drug systemically.² That directness is the technique’s greatest advantage, though it comes with its own set of challenges.³

Once inside the vitreous, a drug clears through two main routes. Some molecules drift forward through the lens and ciliary body, eventually leaving with the fluid that circulates through the front of the eye. Others permeate backward through the retina and get swept away by the blood flow in the choroid, the vascular layer behind the retina. Drugs that exit via that posterior route tend to have shorter half-lives because the choroid offers a large surface area and active transport mechanisms that clear them quickly.⁴ This clearance behavior is why most intravitreal drugs need to be re-injected on a regular schedule.

What Gets Injected and Why

The vast majority of intravitreal injections today deliver anti-VEGF drugs. VEGF is a protein the body uses to stimulate new blood vessel growth, which sounds helpful until those blood vessels sprout in the wrong place. In wet age-related macular degeneration, diabetic macular edema, and retinal vein occlusion, abnormal vessels leak fluid and blood into the retina, destroying vision. Anti-VEGF agents block that process. The most commonly used include ranibizumab, bevacizumab, aflibercept, and newer arrivals like faricimab and brolucizumab.

Meta-analyses comparing these agents in wet macular degeneration have found them broadly similar in outcomes. A systematic review found that ranibizumab and aflibercept produced comparable improvements in visual acuity, retinal thickness, and lesion size, with a small statistical edge for ranibizumab over bevacizumab.⁵ In practical terms, the differences between agents are small enough that the choice often comes down to cost, dosing interval, and individual response.

Corticosteroid implants are the second major category. A dexamethasone implant, sold under the brand name Ozurdex, is a tiny biodegradable rod placed inside the eye. It slowly releases medication over months and is used primarily for noninfectious uveitis (inflammation inside the eye) and macular edema. Studies have shown the implant reduces inflammation for roughly three to six months before retreatment may be needed.⁶ ⁷

How Treatment Schedules Work

One of the biggest practical questions patients face is how often they will need injections. Anti-VEGF therapy for wet macular degeneration typically starts with a loading phase of monthly injections for several months. After that, the schedule depends on the dosing strategy the retina specialist chooses.

Monthly injections were the original standard, but they impose a heavy burden. A treat-and-extend approach has gained wide adoption: the interval between injections is gradually lengthened as long as the disease stays quiet, and shortened if it flares. A meta-analysis found that treat-and-extend dosing achieved visual outcomes statistically indistinguishable from monthly injections while requiring about four and a half fewer injections over the treatment period.⁸ Another common strategy, as-needed (or PRN) dosing, means the patient gets injected only when imaging shows the disease is active. Treat-and-extend results in fewer clinic visits than PRN, because PRN requires monitoring appointments between injections to decide whether another is needed.⁹

Long-term data from real-world clinics show that the treat-and-extend approach preserves independence-level vision over five years of treatment.¹⁰ Still, even with extended intervals, many patients receive dozens of injections over the years. That cumulative burden is not just physical; it also has financial weight. A recent cross-country survey found that the economic burden on patients and caregivers is significant, with costs in the United States falling hardest on caregivers and running into the thousands of dollars over six-month periods. The burden was consistently lower for patients receiving fewer injections.¹¹

What the Procedure Feels Like

The injection itself takes seconds. The eye is numbed with anesthetic drops, the surface is cleaned with povidone-iodine to reduce infection risk, and a very fine needle (typically 30- or 32-gauge) is inserted through the white part of the eye into the vitreous. Most patients report pressure or mild discomfort rather than sharp pain, though the anticipation is often worse than the reality.

The antiseptic step matters more than patients might think. Povidone-iodine applied to the eye surface before injection is the primary defense against infection. A study comparing concentrations of 2.5% and 5% found both equally effective at preventing post-injection infection, which is useful because the lower concentration tends to cause less stinging.¹²

The Anxiety Factor

The idea of a needle entering your eye triggers a visceral reaction, and clinicians are increasingly recognizing the psychological toll that repeated injections take. A narrative review found that anxiety is a common companion to intravitreal treatment, driven by factors including lack of education about the procedure, unfamiliarity, pain, and the logistics of getting to appointments. That anxiety matters clinically because it can reduce adherence, meaning patients skip or delay injections and lose vision as a result.¹³ Clinics that take time to walk patients through what to expect, use a calm and unhurried approach, and address pain management tend to get better adherence over the long haul.

Infection Risk

The most feared complication is endophthalmitis, a severe infection inside the eye. The good news is that it is rare. A study of over 90,000 injections identified 30 cases, putting the rate at roughly 1 in 3,000 injections. Nearly half of those cases were culture-negative, meaning no organism could be grown from the fluid, and the most commonly identified bacteria were coagulase-negative staphylococci, a type that normally lives on the skin.¹⁴ Another large study found a similar rate of about 1 in 5,000 injections, with gram-positive bacteria responsible in the vast majority of culture-positive cases.¹⁵

Symptoms of endophthalmitis usually appear within days and include worsening pain, redness, and a sudden drop in vision. It requires urgent treatment, typically with intravitreal antibiotics and sometimes surgery. While the infection itself is serious, many patients recover usable vision when it is caught early.

Sterile Inflammation

Not all post-injection inflammation is caused by an infection. Sterile inflammation, sometimes called sterile endophthalmitis, occurs when the immune system reacts to the injected drug or its components rather than to bacteria. Rates vary depending on the agent, ranging from about 0.05% to 4.4% of injections. Brolucizumab in particular attracted attention for causing inflammatory vasculitis, a more serious form of sterile inflammation affecting retinal blood vessels, in about 3.3% of injections in clinical trial data.¹⁶ That finding led many clinicians to be more cautious with brolucizumab, especially in patients who had not tried other agents first.

Pressure Spikes After Injection

Injecting fluid into a closed space predictably raises the pressure inside it, and the eye is no exception. Immediately after an intravitreal injection, eye pressure can jump dramatically. A meta-analysis of 46 studies found that pressure rose by an average of about 23 mmHg within the first minute, often exceeding 40 mmHg.¹⁷ For context, normal eye pressure ranges from about 10 to 21 mmHg, so the immediate post-injection spike can briefly push the eye into what would ordinarily be considered dangerously high territory.

Fortunately, the spike is usually short-lived. The same meta-analysis found that pressure had dropped to only about 2.5 mmHg above baseline by 30 minutes and was actually slightly below baseline by the next day, returning fully to normal within a week.¹⁸ For most people, this acute spike passes without consequence. For patients with glaucoma or borderline-high baseline pressure, though, even a brief spike can be more concerning, and clinicians often monitor these patients more closely after injection.¹⁹

Dexamethasone implants tell a different pressure story. Rather than a brief spike, they can cause a sustained rise over weeks to months. One study found that about 19% of eyes receiving the implant had a pressure increase, peaking around two months, with most resolving by six months. Some eyes reached pressures in the mid-30s and needed medication to bring them down, though none required surgery.²⁰ This difference is important: if you are receiving steroid implants, your doctor will want to check your pressure regularly for several months afterward.

Retinal Detachment

The needle passing through the eye wall can, in rare cases, cause a retinal tear or detachment. A large meta-analysis pooling nearly 788,000 injections found the risk of rhegmatogenous retinal detachment (the type caused by a tear) to be about 1 in 8,700 injections, or roughly 0.012%. Looked at per eye rather than per injection, the risk was about 1 in 1,250.²¹ There is one notable exception: in patients with severe proliferative diabetic retinopathy, where scar-like tissue already tugs on the retina, the risk jumps to about 1 in 28, reflecting the underlying disease rather than the injection itself.²²

A case series examining retinal detachments occurring within three months of injection found that most happened when the injection was given in the lower part of the eye, and over a third of affected eyes had pre-existing risk factors like prior eye surgery or a history of retinal tears.²³ For the typical patient without those risk factors, retinal detachment from intravitreal injection is genuinely uncommon.

Systemic Side Effects

Although intravitreal injection is local therapy, small amounts of the drug do make it into the general circulation. For most patients, this systemic absorption appears clinically insignificant. However, anti-VEGF drugs block a protein that the cardiovascular system also relies on for blood vessel maintenance, which raises theoretical concerns about heart attacks and strokes. A review examining safety data across organ systems concluded that while intravitreal anti-VEGF therapies are generally safe, cardiovascular and cerebrovascular risks should not be overlooked in high-risk individuals, particularly those with diabetes or pre-existing heart disease. The choice of agent should be tailored accordingly.²⁴

Intravitreal Injections in Premature Infants

Retinopathy of prematurity, a disease of abnormal blood vessel growth in the eyes of premature babies, is one setting where intravitreal anti-VEGF has increasingly replaced traditional laser treatment. A systematic review comparing the two found that laser therapy leads to higher rates of myopia (nearsightedness) than anti-VEGF injections, while rates of other adverse events and neurodevelopmental outcomes were similar.²⁵ One trial found that a lower dose of an anti-VEGF agent successfully treated the disease in about 84% of eyes without any observed systemic or ocular complications during follow-up.²⁶

The concern unique to this population is what happens to a developing body when VEGF is blocked. VEGF is not only needed for eye development; it plays roles in brain, lung, and kidney maturation. Some studies have reported lower motor skills and possibly increased risk of neurodevelopmental disability in babies receiving bevacizumab compared to laser.²⁷ There is also a recurrence problem: the disease sometimes comes back weeks or months after anti-VEGF treatment, requiring close monitoring and occasionally retreatment. These trade-offs are still being sorted out, and the evidence is not yet settled enough for a blanket recommendation in one direction.

Reducing the Injection Burden

Given the physical, psychological, and financial costs of repeated injections, a great deal of research is aimed at making each treatment last longer. One approach already on the market is the Port Delivery System, a small refillable implant surgically placed in the eye wall. It continuously releases ranibizumab into the vitreous, and in laboratory testing, higher starting concentrations of the drug maintained measurable release for over six months.²⁸ Instead of monthly injections, patients return roughly every six months for a refill exchange procedure. The trade-off is a more involved initial surgery, but for patients who need frequent anti-VEGF treatment, the math can work out in their favor.

Suprachoroidal injection is an alternative delivery route that has also gained traction. Rather than entering the vitreous, the drug is placed in the narrow space between the sclera (the white outer wall) and the choroid. In animal studies, this route achieved peak drug concentrations in the choroid and retina that were roughly 25 times higher than an equivalent intravitreal injection, with the drug arriving almost immediately.²⁹ Suprachoroidal delivery is now approved for certain steroids and is being explored for other drugs, though intravitreal injection remains the dominant route for anti-VEGF treatment.

Gene Therapy Through the Vitreous

The newest frontier for intravitreal injection is gene therapy. The idea is to deliver a gene-carrying virus into the eye that instructs retinal cells to produce their own therapeutic protein continuously, potentially eliminating the need for repeated injections altogether. The challenge is that a membrane called the inner limiting membrane sits between the vitreous and the retina like a biological fence, limiting how well viruses can penetrate. Research in animal models has shown that this barrier can be overcome, with certain viral vectors combined with enzyme pre-treatment achieving efficient delivery to multiple retinal cell types, including the photoreceptors themselves.³⁰ Several gene therapies are in clinical trials now, and if they succeed, the era of monthly eye injections could eventually end for at least some patients.

Translating Animal Data to Humans

Much of what we know about drug behavior inside the vitreous comes from animal studies, primarily in rabbits and mice. But the eye varies enormously across species. Mouse vitreous drug half-lives are three to twelve times shorter than in rabbits, and the dose-scaling factors needed to translate from mouse to human studies are substantial, ranging from roughly 38-fold to 126-fold.³¹ The animal models used to develop intravitreal drugs differ from humans in their vitreous geometry, composition, and clearance pathways.³² This gap means that promising preclinical results do not always translate smoothly into human outcomes, and it partly explains why optimizing doses and intervals for new intravitreal therapies requires large, careful human trials.