Carbonic anhydrase inhibitor + beta-blocker (combination eye drops) Pregnancy: Should not be used during pregnancy (no adequate clinical data; dorzolamide teratogenic in rabbits at maternotoxic doses; timolol should not be used unless clearly necessary). If administered until delivery, monitor the neonate carefully during the first days of life. Lactation is not recommended if treatment is required (§4.6)

Dorzolamide with timolol

Brand names: Cosopt

This is a topical ophthalmic fixed combination of dorzolamide (a carbonic anhydrase inhibitor) and timolol (a non-selective beta-blocker) used to reduce intraocular pressure in open-angle glaucoma and ocular hypertension where monotherapy is insufficient.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: One drop in the (conjunctival sac of the) affected eye(s)

Route: Ocular (topical eye drops, solution)

Frequency: Two times daily

Source SPC is COSOPT 20 mg/ml + 5 mg/ml eye drops, solution (dorzolamide 20 mg/ml + timolol 5 mg/ml) — the exact fixed combination. §4.2: 'The dose is one drop of COSOPT in the (conjunctival sac of the) affected eye(s) two times daily.' If another topical ophthalmic agent is being used, COSOPT and the other agent should be administered at least ten minutes apart. Administration: wash hands; do not let the dropper tip touch the eye, surrounding skin or fingers; after instilling, close the eye and press the inner corner of the eye for about two minutes — nasolacrimal occlusion or eyelid closure for 2 minutes reduces systemic absorption, which may decrease systemic side effects and increase local activity. Paediatric (§4.2): 'Efficacy in paediatric patients has not been established. Safety in paediatric patients below the age of 2 years has not been established.' Systemic beta-blockade effects can occur because timolol is absorbed systemically; dorzolamide contains a sulfonamido group so sulfonamide-type reactions (including Stevens-Johnson syndrome and toxic epidermal necrolysis) may occur. Not studied in hepatic impairment — use with caution.

Dose adjustments

Renal

Contraindicated in severe renal impairment (CrCl < 30 ml/min) (§4.3). No other renal dose adjustment is stated in the source

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Reactive airway disease, including bronchial asthma or a history of bronchial asthma, or severe chronic obstructive pulmonary disease
Sinus bradycardia, sick sinus syndrome, sino-atrial block, second or third degree atrioventricular block not controlled with pacemaker, overt cardiac failure, cardiogenic shock
Severe renal impairment (CrCl < 30 ml/min) or hyperchloraemic acidosis
Hypersensitivity to one or both active substances or to any of the excipients

Side effects

Very common/common with the combination: burning and stinging on instillation
Common: conjunctival injection, blurred vision, corneal erosion, ocular itching, tearing
Signs and symptoms of systemic allergic reactions, including angioedema, urticaria, pruritus, rash, anaphylaxis
With dorzolamide component: headache, dizziness, paraesthesia; eyelid inflammation/irritation; iridocyclitis; transient myopia
With timolol component: bradycardia and other systemic beta-blockade effects, hypoglycaemia, depression/insomnia/nightmares, syncope, blepharitis, keratitis, decreased corneal sensitivity, dry eyes
Approximately 2.4% of patients discontinued because of local ocular adverse reactions and 1.2% because of local reactions suggestive of allergy/hypersensitivity

Interactions

Systemic beta-blocking agents — the effect on intra-ocular pressure or the known effects of systemic beta-blockade may be potentiated when timolol is given to patients already receiving a systemic beta-blocker (§4.4 Concomitant Therapy; eMC text truncated in source bundle — confirm full §4.5 against the SPC)
Other topical ophthalmic agents — administer at least ten minutes apart (§4.2)
Adrenaline used to treat anaphylaxis — patients on beta-blockers with a history of atopy or severe anaphylactic reaction may be more reactive to repeated allergen challenge and unresponsive to the usual adrenaline dose (§4.4)
Oral carbonic anhydrase inhibitors — concomitant administration not recommended (potential additive systemic carbonic anhydrase inhibition) [US dorzolamide label §7.1 — verify against UK SPC §4.5]
High-dose salicylate therapy — potential acid-base and electrolyte disturbances [US dorzolamide label §7.2 — verify against UK SPC §4.5]

Clinical monograph

How it works

Dorzolamide decreases aqueous humour secretion via inhibition of ciliary carbonic anhydrase, and timolol reduces aqueous production through beta-adrenergic blockade, giving additive intraocular pressure reduction.

Prescribing in practice

Because the timolol component is systemically absorbed, the combination is contraindicated in reactive airways disease (asthma, severe COPD), sinus bradycardia, heart block and uncontrolled heart failure.
Avoid in patients with sulphonamide hypersensitivity, as dorzolamide is a sulphonamide derivative.
Use cautiously in severe renal impairment, where systemic dorzolamide accumulation and metabolic acidosis are more likely.

Monitoring

Check intraocular pressure to confirm response and remain alert to systemic beta-blockade, including bradycardia, bronchospasm and blunting of hypoglycaemic warning signs.

Counselling the patient

Apply gentle pressure to the inner corner of the eye after instilling to minimise systemic absorption.
Separate this from any other eye drops by a few minutes and instil this preparation last if it is a gel-forming product.
Seek advice if you develop breathing difficulty, a slow pulse or marked tiredness.

Evidence & guidelines

NICE glaucoma guidance (NG81) supports combination topical therapy when a single intraocular-pressure-lowering agent fails to achieve target pressures.

Reference: NICE NG81; RCOphth; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways