Ocular pharmacology: getting drugs into the eye

How eye drops, injections and implants reach the eye, why some drops act on the whole body, and how pills can harm the eye.

Updated Nov 12, 2026

The eye is small, well protected and partly walled off from the blood, so most eye drugs are given locally. The route decides which part of the eye a drug reaches: drops treat the surface and the front chamber, while the retina usually needs an injection or an implant. The same anatomy explains why eye drops can cause body-wide side effects and why some systemic drugs damage the eye.

Key points

The tear film, the cornea and blinking limit how much of a drop gets in; much of each drop drains away through the nasolacrimal duct.
Drops that drain into the nose are absorbed by the nasal mucosa without a first pass through the liver, so a beta-blocker drop such as timolol can cause bronchospasm or a slow pulse.
The cornea works like a fat-water-fat sandwich: a drug with both water- and fat-loving properties crosses it best.
The back of the eye is reached by intravitreal injection or implant; this bypasses absorption but carries a risk of endophthalmitis.
Topical glucocorticoids can cause cataract, raised eye pressure and worse infection; topical NSAIDs are an alternative for some postoperative inflammation.
Systemic drugs can harm every part of the eye: hydroxychloroquine (retina), ethambutol (optic nerve), amiodarone (cornea), topiramate (angle closure), tamsulosin (floppy iris in surgery).

Anatomy that matters for drug delivery

Tears constantly wash the surface and drain through the puncta into the nasolacrimal duct and the nose. The cornea has three layers that act as barriers in turn: the epithelium and endothelium block water-soluble drugs, the watery stroma between them blocks fat-soluble ones. Behind the lens, the vitreous and the blood–retinal barrier keep most drugs from the blood away from the retina. The eye also contains enzymes, such as esterases, that can activate prodrugs: latanoprost is activated in the eye, and loteprednol is inactivated there, which limits its effects elsewhere.

Routes and how long a drug stays

Topical drops, gels and ointments: for the surface and the front of the eye; thicker vehicles and inserts keep the drug in contact longer.
Punctal plugs or gentle pressure on the inner corner of the eye slow drainage, keeping more drug in the eye and less in the body.
Subconjunctival, sub-Tenon and peribulbar or retrobulbar injections: deliver drugs around the globe, for example steroids for posterior uveitis or local anaesthesia for surgery.
Intracameral injection: into the front chamber during surgery.
Intravitreal injection or implant: for the retina, as with anti-VEGF antibodies, steroids or ganciclovir; absorption is bypassed but infection, retinal toxicity and tissue injury are possible.

Treating eye infections

Bacterial conjunctivitis, keratitis and blepharitis are treated mainly with topical antibiotics such as fluoroquinolones, aminoglycosides and macrolides. Herpes simplex epithelial keratitis is treated with topical or oral antivirals, and topical steroids are avoided in that form because they worsen viral replication. For fungal keratitis, natamycin, a polyene, is the only commercially available topical eye antifungal, and other antifungals are compounded. Endophthalmitis, an infection inside the eye, usually follows surgery or trauma; it is an emergency treated with a vitreous sample and antibiotics injected straight into the vitreous.

Anti-inflammatory, antiallergic and antimitotic drugs

Glucocorticoids (dexamethasone, prednisolone, fluorometholone, loteprednol, difluprednate) treat uveitis, allergy and postoperative inflammation; they can cause posterior subcapsular cataract, steroid-induced glaucoma and worse herpetic, fungal or Acanthamoeba infection.
Topical NSAIDs (ketorolac, diclofenac, bromfenac, nepafenac, flurbiprofen) reduce pain, inflammation and cystoid macular oedema after cataract surgery; rarely they cause corneal melting in dry, diseased eyes.
Antihistamine and mast-cell-stabiliser drops treat allergic conjunctivitis; ketorolac drops are also used for seasonal allergic conjunctivitis.
Ciclosporin and lifitegrast drops treat inflammatory dry eye.
Mitomycin and fluorouracil, both antineoplastic, are applied during glaucoma filtering surgery to limit scarring; mitomycin also reduces pterygium recurrence, at the price of possible delayed complications.

Agents used during eye surgery

Viscoelastic substances, made from hyaluronate, chondroitin sulfate or hydroxypropyl methylcellulose, keep spaces open, move tissue and protect the corneal endothelium; if left behind they can raise eye pressure after surgery. Trypan blue stains the lens capsule so it can be seen during cataract surgery. Mydriatics keep the pupil wide, miotics close it at the end, and local anaesthetics are given as drops or injections. Tissue adhesives and fibrin sealants help close corneal perforations and secure grafts.

Ocular side effects of systemic drugs

Retina: hydroxychloroquine and chloroquine cause a lasting retinopathy related to cumulative exposure; the label recommends a baseline exam in the first year and yearly screening, started early in people at higher risk.
Optic nerve: ethambutol, chloramphenicol and rifampicin can cause toxic optic neuropathy; PDE5 inhibitors weakly block PDE6 in the retina and tint vision blue.
Pressure and angle: topiramate can cause acute angle-closure glaucoma; steroids by any route can raise eye pressure.
Lens and cornea: steroids cause cataract; amiodarone leaves whorl-shaped corneal deposits that rarely affect vision; phenothiazines deposit brown pigment.
Surgery: alpha-1 blockers such as tamsulosin cause intraoperative floppy iris syndrome, so the eye surgeon needs to know about them before cataract surgery.

Neurotrophic keratopathy

The cornea is the most densely innervated tissue in the body, and its nerves keep the epithelium healthy. When they are damaged by herpes infection, chemical burns, long-standing diabetes, surgery or a brain lesion, the cornea loses feeling and epithelial defects stop healing. Classic care uses lubricants, autologous serum drops, eyelid closure or amniotic membrane. Cenegermin, a recombinant human nerve growth factor given as eye drops, was approved in the US in 2018 specifically for this condition.

Test yourself

Think of the answer, then tap the question to see it.

Why can timolol eye drops cause bronchospasm in a person with asthma?

The drop drains through the nasolacrimal duct and is absorbed by the nasal mucosa straight into the blood, without a first pass through the liver, so it blocks beta-2 receptors in the airways.

What kind of drug crosses the cornea best, and why?

One with both water- and fat-soluble properties, because the epithelium and endothelium block water-soluble drugs while the watery stroma between them blocks fat-soluble ones.

Why are topical steroids avoided in herpes simplex epithelial keratitis?

They favour viral replication and can make the corneal infection worse; antivirals are the treatment for that form.

Which drug taken by mouth should a cataract surgeon know about because it can cause a floppy iris during surgery?

Tamsulosin and other alpha-1 blockers, which cause intraoperative floppy iris syndrome and raise the risk of complications.

What does cenegermin treat and how does it work?

Neurotrophic keratopathy: it is a recombinant human nerve growth factor given as eye drops that supports the corneal nerves and epithelial healing.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - TIMOLOL MALEATE- timolol solution/ drops (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - HYDROXYCHLOROQUINE SULFATE tablet (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - TAMSULOSIN HYDROCHLORIDE capsule (DailyMed (U.S. National Library of Medicine), 2026).

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