Instillation in Medicine: How Local Drug Delivery Works

Instillation is the slow, controlled delivery of a liquid medication or solution directly into a body cavity, where it acts locally rather than traveling through the bloodstream first. The technique is used across nearly every branch of medicine, from the eye drops you squeeze in for allergies to chemotherapy dripped through a catheter into the bladder. What makes instillation distinct from injection or oral dosing is its defining advantage: the drug reaches the target tissue at high concentration while sparing the rest of the body from most side effects. That trade-off between local potency and systemic gentleness explains why instillation keeps showing up in contexts where oral or intravenous drugs fall short.

Why Deliver Drugs Locally Instead of Systemically

The logic behind instillation is straightforward. When you swallow a pill, the drug dissolves in your stomach, enters the bloodstream, circulates everywhere, and only a fraction reaches the organ that actually needs it. Along the way, it can cause side effects in tissues that never asked for it. Instillation flips that equation. By placing the drug directly into the bladder, the eye, the lung, or another enclosed space, clinicians can expose the diseased tissue to much higher drug concentrations than an oral dose could ever achieve there, while keeping blood levels low enough to avoid the worst systemic reactions. For cancers confined to the bladder lining, for instance, intravesical delivery allows drug concentrations at the tumor site that would be toxic if those same levels circulated through the whole body.

Eye Drops and the Challenges of Ophthalmic Instillation

The form of instillation most people encounter in daily life is the humble eye drop. Glaucoma medications, antibiotic drops for conjunctivitis, and lubricating artificial tears are all instilled onto the surface of the eye. Despite how routine eye drops seem, getting a drug to actually penetrate the eye is surprisingly difficult. The eye has an arsenal of defenses: blinking sweeps the drop away within seconds, tears dilute it, and nasolacrimal drainage channels the liquid down into the nose and throat before much absorption can happen. On top of that, the cornea’s tightly packed outer cell layer resists drug diffusion, and enzymes on the ocular surface can break down active ingredients before they reach deeper structures.¹

The result is that a very small percentage of each drop’s active ingredient actually reaches the interior of the eye. One way researchers have tried to improve on this is by reformulating drugs as solid wafer-like inserts (lyophilisates) that dissolve slowly on the eye surface. In studies comparing a lyophilisate form of fluorescein to conventional drops, the solid form produced peak drug concentrations in the cornea and front chamber of the eye roughly six to fifteen times higher, with overall drug exposure about five to nine times greater.² The longer contact time let more drug soak in before tears washed it away.

Even with standard drops, technique makes a real difference in how much drug gets absorbed, and how much drains systemically into the bloodstream. Pressing the inner corner of the eye after instilling a drop (nasolacrimal occlusion) blocks the drainage duct and keeps the medication on the eye longer. A clinical trial testing timolol, a glaucoma drug known for systemic side effects like slowed heart rate, found that plasma drug levels dropped significantly when patients used either nasolacrimal occlusion or a simpler “tissue press” method compared to no intervention at all.³ For a drug that can affect the heart, reducing how much leaks into the bloodstream through the nose matters.

Most People Use Eye Drops Poorly

If instillation technique matters so much for eye drops, you might expect patients to be carefully trained. In practice, most are not. A real-world study that observed people actually instilling their eye drops found that while nearly everyone managed to get at least one drop into the eye, only about 3% performed the full technique correctly. The most common mistakes were touching the bottle tip to the eye or eyelid (about 41% of patients), failing to close the eye afterward (roughly 68%), and skipping nasolacrimal occlusion almost entirely (about 95%).⁴ About 40% of patients reported at least one problem with the process, including difficulty aiming the drop, too many drops coming out at once, and trouble squeezing the bottle.⁵

These aren’t minor annoyances. For someone with glaucoma, where daily medicated drops are the main treatment preventing vision loss, poor technique means less drug reaches the eye and more is wasted or absorbed systemically. Contaminating the bottle tip by touching it to the eye can also introduce bacteria. Various drop-dispensing aids exist to help with aiming and squeezing, but adoption remains low, partly because many patients don’t realize their technique is flawed in the first place.

Instillation Into the Bladder

Intravesical instillation, where a solution is delivered through a catheter directly into the bladder, is one of the most established uses of the technique. The bladder is essentially a sealed muscular bag, making it ideal for holding a drug bath against diseased tissue. Two major applications dominate: cancer treatment and management of chronic bladder pain.

For non-muscle-invasive bladder cancer, the standard approach after tumor removal is to instill BCG (Bacillus Calmette-Guérin), a live weakened bacterium originally developed as a tuberculosis vaccine. BCG works by triggering a potent local immune response inside the bladder. The bacterium activates both the fast-acting innate immune system and the slower, more targeted adaptive immune system, ultimately directing immune cells to attack remaining cancer cells.⁶ More recent research has uncovered additional mechanisms: BCG can actually get inside cancer cells and turn them into beacons that attract immune attack, it can directly kill cancer cells through pathways that trigger cell death, and it can modulate immune-checkpoint molecules on both tumor and immune cells.⁷ This last effect is a double-edged sword, since the same checkpoint modulation that helps the immune system in some contexts can also dampen it, which partly explains why some patients respond better to BCG than others.

Another intravesical chemotherapy agent, mitomycin C, illustrates how much the details of instillation matter. Pharmacokinetic modeling showed that the drug breaks down in acidic urine and gets diluted by any urine the bladder keeps producing during the treatment dwell period. Of all the variables that affected how much drug actually reached the tissue, dose was the most important, followed by how completely the bladder was emptied beforehand and how much urine was produced during treatment. Even something as simple as having the patient limit fluid intake before the session or alkalinizing the urine to a neutral pH could meaningfully boost drug exposure. Extending the dwell time beyond two hours, on the other hand, provided minimal additional benefit and made the process harder for patients to tolerate.⁸

Beyond cancer, intravesical instillation of hyaluronic acid (sometimes combined with chondroitin sulfate) is used to treat interstitial cystitis and bladder pain syndrome, conditions marked by chronic pelvic pain, urinary urgency, and frequent urination. The idea is to replenish the protective glycosaminoglycan layer that lines the bladder wall and becomes damaged in these conditions. A meta-analysis of the approach found significant improvements in pain scores, symptom indices, and bladder capacity.⁹ Longer follow-up data from a three-year study showed that these improvements were sustained, with daily voiding frequency dropping from about 18 times a day at baseline to roughly 12 by year three, and the volume per void increasing from about 137 mL to 181 mL.¹⁰ For patients who have already failed other therapies, repeated instillation sessions can offer meaningful relief.¹¹

Surfactant Instillation in Premature Infants

One of the most dramatic uses of instillation is in neonatology. Premature babies often lack surfactant, the slippery substance that coats the inside of the lungs and keeps the tiny air sacs from collapsing with each breath. Without it, they develop respiratory distress syndrome, which can be fatal. The treatment is to instill liquid surfactant directly into the windpipe, where it spreads through the lungs and restores their ability to expand.

A landmark randomized trial showed that surfactant instillation before the first breath in premature infants significantly improved gas exchange over the first 72 hours, cut the need for supplemental oxygen roughly in half, and reduced the incidence of pulmonary air leaks (interstitial emphysema) from about 39% to about 8%. Neonatal deaths dropped from six out of 33 untreated infants to one out of 39 treated infants.¹² A Cochrane review later confirmed that giving surfactant early, followed by gentle non-invasive breathing support, cut the need for prolonged mechanical ventilation by about half compared to waiting and treating selectively.¹³

Even the mechanics of how surfactant is instilled can affect outcomes. Animal studies in premature primates showed that maintaining positive pressure in the airway during the instillation, rather than briefly disconnecting the ventilator to squirt the liquid in, resulted in less oxygen desaturation.¹⁴ That kind of detail, something as seemingly minor as whether the baby stays connected to the ventilator for an extra few seconds, can determine how evenly the surfactant distributes through the lungs.

Instillation Into the Chest and Abdomen

When fluid accumulates in the space between the lungs and the chest wall, and that fluid becomes organized into pockets separated by fibrin strands, a simple chest drain often can’t pull it out. Intrapleural instillation of fibrinolytic enzymes offers an alternative to surgery. The instilled enzymes break apart the fibrin clots that form the internal walls of these pockets, freeing the trapped fluid so it can drain. Beyond clot-busting, the treatment also has anti-inflammatory properties that help reduce ongoing inflammation in the pleural space.¹⁵ This makes it a valuable option for patients who aren’t good candidates for surgery, or when a chest tube alone isn’t getting the job done.¹⁶

In the abdomen, the concept is taken further with heated intraperitoneal chemotherapy (HIPEC). During surgery for cancers that have spread to the lining of the abdominal cavity, surgeons remove all visible tumor and then bathe the open abdomen in a warm chemotherapy solution. The heat itself selectively kills cancer cells while also boosting the penetration and potency of the chemotherapy agents. Because the drug stays within the abdominal cavity, regional drug concentrations can be pushed far higher than intravenous dosing would safely allow.¹⁷ HIPEC is a major procedure used primarily for cancers like peritoneal mesothelioma and advanced ovarian or colorectal cancers with peritoneal spread, and it represents one of the most aggressive forms of instillation in modern practice.

Instillation Through a Bronchoscope

A more targeted form of pulmonary instillation involves threading a bronchoscope into the airways and delivering a drug directly to a diseased area of the lung. This has found a niche in treating chronic cavitary pulmonary aspergillosis, a fungal infection that hollows out cavities in the lung and can be stubbornly resistant to oral antifungals. By instilling amphotericin B directly into the cavity through a bronchoscope, clinicians can achieve drug concentrations at the infection site more than a thousand-fold higher than the minimum needed to kill the fungus, far exceeding what oral or intravenous dosing can deliver there.¹⁸ For patients who aren’t surgical candidates, particularly those with lung cancer, combining local instillation with systemic antifungals has shown promise as an alternative approach.

Ear Drops and Safety Concerns With Otic Instillation

Ear drops are another everyday form of instillation, used for infections, wax softening, and pain relief. What most people don’t realize is that when the eardrum has a hole in it, whether from infection, injury, or a surgically placed tube, the drops don’t just sit in the ear canal. They can flow through into the middle ear and potentially reach the inner ear by crossing the round window membrane. This raises genuine safety concerns, because several ingredients commonly found in otic drops, including certain antibiotics (aminoglycosides), solvents, and preservatives, have known potential to damage the delicate hair cells responsible for hearing and balance.¹⁹ In practice, clinicians weigh the risk of ototoxicity against the risk of leaving an ear infection untreated, and newer fluoroquinolone-based ear drops have become preferred partly because they avoid the aminoglycoside issue.

Instillation for Diagnosis, Not Just Treatment

Instillation isn’t always about delivering drugs. It also plays a role in medical imaging. CT cystography, for example, involves instilling contrast dye through a catheter into the bladder to evaluate suspected bladder injuries after trauma. The contrast fills the bladder and makes tears or ruptures visible on a CT scan, allowing doctors to classify the type and severity of the injury and decide whether surgery is needed, all with less radiation exposure than older imaging methods.²⁰ Similar contrast-instillation techniques are used in the uterus (hysterosalpingography to check fallopian tubes) and in the joints (arthrography to visualize cartilage damage).

New Materials That Extend How Long Instilled Drugs Stay Put

One of the fundamental limitations of instillation is that the body tries to flush the drug away. Tears wash out eye drops, urine dilutes bladder treatments, and mucus clears substances from the airways. Researchers are working on materials that resist this washout effect. In the bladder, hydrogel-based drug carriers are being developed that can adhere to the bladder lining or float in urine, maintaining drug contact with the tissue for hours or even days rather than the one or two hours a liquid solution typically stays put. These materials minimize drug dilution by urine and enhance both absorption and retention time.²¹

The appeal is obvious. If a bladder cancer patient currently needs to hold a chemotherapy solution in their bladder for two hours while lying still, a slow-release hydrogel that sticks to the wall and releases drug over 24 hours could improve treatment effectiveness and dramatically reduce the burden on the patient. Similar thinking drives the development of slow-dissolving ocular inserts, nasal gels, and thermosensitive solutions that become semi-solid at body temperature after being instilled as a liquid. The shared goal across all these innovations is the same: keep the drug where it was put, for longer, without requiring the patient to do anything special.

For a technique that dates back centuries in concept, instillation keeps finding new territory. The underlying principle hasn’t changed: put the medicine where the disease is. What continues to evolve is how creatively clinicians and engineers manage to keep it there.