Pearls from the EGS Guidelines
Publishing date: September 2020
Pearls Manager: Andrew Tatham
When two eye drops are instilled 30 seconds apart, almost 50% of the first drug will be washed out. It is recommended that patients wait at least 5 minutes between drops, which reduces the washout effect to less than 15%. Typically, less than 5% of the volume of an eye drop enters the eye, with the rest running down the cheek or entering the nasolacrimal duct. There is no evidence that digital nasolacrimal duct occlusion increases efficacy of topical medications however it may reduce systemic side effects, particularly from beta-blockers, by minimising drainage into the highly vascularised nasopharyngeal mucosa.
EGS Guidelines, 4th edition, Page 157.
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2. Linden C, Alm A. The effect of reduced tear drainage on corneal and aqueous concentrations of topically applied fluorescein. Acta Ophthalmol 1990;68(6):633-8.