Dorzolamide / Timolol Fixed Combination (Cosopt)

How it works

Dorzolamide reduces aqueous humour production by inhibiting carbonic anhydrase in the ciliary epithelium, while timolol further suppresses aqueous formation through beta-adrenergic blockade; the two complementary mechanisms produce additive pressure lowering.

Prescribing in practice

• The timolol component carries systemic beta-blockade risk on absorption, so it is contraindicated in asthma, a history of bronchospasm, severe COPD, sinus bradycardia, second- or third-degree AV block and overt heart failure.
• Dorzolamide is a sulphonamide derivative and may cause hypersensitivity reactions in patients sensitised to sulphonamides.
• Nasolacrimal occlusion or gentle eyelid closure after instillation reduces systemic absorption of both components.

Monitoring

Monitor intraocular pressure response and review for systemic beta-blocker effects such as bradycardia, bronchospasm and masked hypoglycaemia, particularly in at-risk patients.

Counselling the patient

• Remove soft contact lenses before instilling and wait before reinserting, as the preservative can be absorbed by lenses.
• Press gently on the inner corner of the eye for a minute after each drop to limit drug reaching the rest of the body.
• Report wheeze, breathlessness, a slow heartbeat or unusual fatigue.

Evidence & guidelines

Fixed combinations of dorzolamide and timolol are established second-line options in NICE guidance on glaucoma (NG81) when a single topical agent does not control intraocular pressure.