Why is it important to take your eye medications as prescribed?

For people with glaucoma, missing or mistiming prescribed eye drops leads to irreversible vision loss (Sleath et al., 2011). In a study evaluating glaucoma patients over an average of 7.3 years, researchers found a dose-response relationship between missing medication and the deterioration of a patient's visual field (Newman-Casey et al., 2020).

Why is it necessary to wait at least 5 minutes between instilling eye drops prescribed at the same time? 

It is due to the physiological limitations of the human ocular surface and the pharmacokinetics of topical drug absorption. When a second drop is instilled shortly after the first, it triggers an immediate overflow. The excess fluid is rapidly evacuated via the nasolacrimal duct (Agrahari et al., 2016) and physiological tear turnover (Agarwal et al., 2021). Consequently, the second drop physically "washes out" the first before it has had sufficient time to penetrate the cornea. Waiting at least 5 minutes has been scientifically proven to make each medication more clinically effective (Denion et al., 2017). 

Eyedrop identification by bottle cap color

Beige: antibiotics
Blue: selective beta-blockers or combination of beta-blocker and carbonic anhydrase inhibitor - lowers intraocular pressure by reducing fluid production
Gray: Nonsteroidal Anti-inflammatory Drug - reduces inflammation
Green: cholinergic receptor agonist - constricts pupil and lowers intraocular pressure by increasing outflow
Light Green: combination of alpha-agonist and carbonic anhydrase inhibitor – lowers intraocular pressure by reducing fluid production and increasing outflow
Orange: Carbonic anhydrase inhibitors – lowers intraocular pressure by reducing fluid production
Pink: steroids – reduces inflammation 
Purple: alpha-agonists – lowers intraocular pressure by reducing fluid production and increasing outflow
Red: cycloplegics – dilates the pupil and relaxes the ciliary muscle
Turquoise: prostaglandin analogs – lowers intraocular pressure by increasing outflow
White: steroids – reduces inflammation
Yellow: beta-blockers – lowers intraocular pressure by reducing fluid production

REFERENCES:
Newman-Casey, P. A., Niziol, L. M., Gillespie, B. W., Janz, N. K., Lichter, P. R., & Musch, D. C. (2020). The association between medication adherence and visual field progression in the Collaborative Initial Glaucoma Treatment Study. Ophthalmology, 127(4), 477–483. https://doi.org/10.1016/j.ophtha.2019.10.022

Sleath, B., Blalock, S., Covert, D., Stone, J. L., Skinner, A. C., Muir, K., & Robin, A. L. (2011). The relationship between glaucoma medication adherence, eye drop technique, and visual field defect severity. Ophthalmology, 118(12), 2398–2402. https://doi.org/10.1016/j.ophtha.2011.05.013

Agrahari, V., Mandal, A., Agrahari, V., Trinh, H. M., Joseph, M., Ray, A., Hadji, H., Mitra, R., Pal, D., & Mitra, A. K. (2016). A comprehensive insight on ocular pharmacokinetics. Drug Delivery and Translational Research, 6, 735-754. https://doi.org/10.1007/s13346-016-0339-2

Agarwal, P., Craig, J. P., & Rupenthal, I. D. (2021). Formulation Considerations for the Management of Dry Eye Disease. Pharmaceutics, 13(2), 207. https://doi.org/10.3390/pharmaceutics13020207

Denion, E., Charlot, F., & Béraud, G. (2017). A 5-Minute Interval between Two Dilating Eye Drops Increases Their Effect. Optometry and Vision Science, 94(8), 838-844. https://doi.org/10.1097/opx.0000000000001104