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Fixed Combination Glaucoma Treatment — CAI + Beta-blocker Pregnancy: Avoid — timolol crosses placenta; may cause fetal bradycardia

Dorzolamide / Timolol Fixed Combination (Cosopt)

Brand names: Cosopt, Cosopt PF (preservative-free)

Adult dose

Dose: 1 drop twice daily
Route: Topical (ophthalmic)
Frequency: Twice daily
Max: 1 drop twice daily
Contains dorzolamide 2% (carbonic anhydrase inhibitor) + timolol 0.5% (beta-blocker). Fixed combination improves compliance vs separate drops. Wait at least 5 minutes between different eye drops. Preservative-free unit-dose version (Cosopt PF) available for patients with ocular surface disease or sensitivity to benzalkonium chloride (BAK).

Paediatric dose

Route: Topical
Frequency: Twice daily
Max: Same as adult
Paediatric glaucoma — significant systemic beta-blocker absorption risk in children; use with caution; avoid in asthma

Dose adjustments

Renal

Avoid if eGFR <30 mL/min — dorzolamide component accumulates

Hepatic

Dorzolamide: use with caution in hepatic impairment

Clinical pearls

  • Fixed combination compliance advantage: patients using multiple single-agent glaucoma drops have significantly higher non-compliance rates — Cosopt as single bottle replaces two separate drops and reduces BAK exposure
  • Systemic beta-blocker absorption from timolol eye drops: the dose absorbed systemically can equal or exceed oral beta-blocker doses — bronchospasm in asthmatics has been fatal; MHRA warning applies to ALL topical timolol preparations
  • Sulfonamide cross-reactivity with dorzolamide: dorzolamide is a sulfonamide derivative — check sulfonamide allergy history; cross-reactivity with other sulfonamides (trimethoprim-sulfamethoxazole) is possible but low risk
  • Bitter taste: virtually universal complaint from dorzolamide — caused by nasolacrimal absorption; nasolacrimal occlusion after instillation reduces systemic absorption and bitter taste simultaneously
  • Nasolacrimal punctum occlusion reduces systemic beta-blocker exposure by approximately 50% — simple technique, particularly important in children and elderly cardiorespiratory patients

Contraindications

  • Asthma, COPD, or history of bronchospasm — timolol contraindicated
  • Bradycardia or heart block — timolol
  • eGFR <30 mL/min — dorzolamide
  • Hypersensitivity to sulfonamides — dorzolamide component

Side effects

  • Bitter taste (nasolacrimal drainage of dorzolamide component — most common complaint)
  • Ocular burning and stinging
  • Corneal epithelial toxicity (BAK in preserved formulation)
  • Systemic beta-blocker effects — bronchospasm, bradycardia, reduced exercise tolerance, masking hypoglycaemia in diabetics
  • Sulfonamide allergy (dorzolamide)

Interactions

  • Systemic beta-blockers — additive effect; AV block, hypotension risk
  • Calcium channel blockers — additive cardiac depression with timolol
  • Systemic carbonic anhydrase inhibitors (acetazolamide) — additive risk of electrolyte imbalance (hypokalaemia, acidosis)

Monitoring

  • IOP at 4–8 weeks
  • Pulse and blood pressure
  • Respiratory symptoms — bronchospasm
  • Corneal surface (BAK effect — switch to PF version if symptomatic)

Reference: BNFc; BNF 90; NICE NG81 (Glaucoma); MHRA Timolol Safety Warning; SPC Cosopt / Cosopt PF. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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