Do you need to wait five minutes between administering eyedrops? Does the second drop really wash out the first? That’s the dogma (1), but is there concrete evidence to support it?
It’s intuitive that if a drug isn’t diluted and washed away by another, its absorption, and action, will be greater, and topically administered drugs do take time to be absorbed. On the other hand, not having to wait between eyedrop instillations would be considerably more convenient for patients – and should reduce the frequency of missed doses because, for example, the patient didn’t have 20 minutes to spare that morning to take four eyedrops, or their mind wandered between drops three and four. Is the wait really that necessary?
To answer this question, a team from Centre Hospitalier Universitaire (CHU) de Caen in France tested the effect of a five-minute time interval between the administration of two different topical mydriatic agents (2). In 20 volunteers (40 eyes), they applied one drop of 10% phenylephrine and added one drop of 0.5% tropicamide immediately afterwards, or following a five-minute interval. Using digital photographs taken in photopic conditions, two observers compared pupil-to-iris surface ratios, finding that the relative pupil surface area was significantly increased with the five-minute wait (observer 1, p=0.004; observer 2, p=0.006) compared with the serial, immediate administration of the topical mydriatics.
The team therefore confirmed that convenience cannot trump pharmacokinetics: if patients are instilling multiple drops, they need to “take a break” of at least five minutes between them.
- SS Chrai et al., “Drop size and initial dosing frequency problems of topically applied ophthalmic drugs”, J Pharm Sci, 63, 333–338 (1974). PMID: 4820359.
- P Saguet et al., “A 5-minute time interval between two different dilating eyedrops increases their combined effect”, Poster presented at the European Association for Vision and Eye Research; October 6, 2016; Nice, France. Poster #T108.
