Look this question up and you will find the same sentence repeated five times on five different sites: a contact lens acts like a sponge for preservatives. It will be repeated by manufacturers, by websites, and by clinical authorities, and not one of them will name the particular preservative or differentiate between hydrogel and silicone hydrogel or provide a single measurement.
The numbers have been recorded.
They are older and more nuanced than the quotation implies, offering contradictory information at once, which is likely the reason why they are not shared at all. Here is what has been measured, what has not, and what you should consider when in a pharmacy at 8pm, your lenses already in, a bottle in your hand.
The rule first, the reasoning after
Only a drop with instructions for use over lens goes in while lenses are in.
All other drops come out first.
That is not a qualification, it is the entire rule, and the several hundred words that follow exist to explain this line and the nuance around it, but your optometrist knows which exceptions exist: they know your material, your wearing schedule, and your ocular surface, none of which any website can see.
Benzalkonium chloride, and the study everyone gestures at
The preservative at the centre of this issue is benzalkonium chloride or BKC, an antimicrobial detergent found in the majority of multidose eye drop bottles for decades, and the reason why you have found yourself researching this topic at all.
An early 1990 in vitro study in Archives of Ophthalmology used radioactively tagged BKC to track its absorption into and release from different contact lens polymers, finding that after seven days of continuous exposure, soft lenses had taken up between 30-56 micrograms of BKC per mg of polymer, and released between 0.2-1.5% of this back into solution during a 24 hour washout period (PubMed).
Higher water content lenses absorbed more, while rigid lenses saw lower overall uptake but significantly higher (30-60%) release with the study authors concluding that the levels released were at or above the upper limits of safety.
That is a definitive claim, and reading the study closely narrows the claim considerably. First and foremost, this is a study that concerns no wearer at all: the lens were soaked in tracer solution for seven days with no blink rate or tear dynamics to wash anything away, and the second qualification is that the polymers tested are older: there were no silicone hydrogels in 1990 and the study does not reference any modern materials. A third qualification is the most difficult one to measure: how much of this gets onto the eye in the first place, and whether that amount falls within the tolerable range or not.
What a newer bench study adds, and where it stops
The research is ongoing, and the most recent study to have appeared in 2021 in Colloids and Surfaces B: Biointerfaces concerns itself with the binding characteristics of BKC and the resulting impact on the mechanical and optical properties of the lens. This study found a maximum adsorption capacity of 2.88 mM for a PVP-free hydrogel and 2.32 mM for a PVP-containing hydrogel with very similar binding constants for both polymers (PubMed).
Physical changes were also seen, including changes to the refractive power, transmittance, contact angle, water content, base curve and diameter of the test lenses, with the authors concluding that PVP-containing lenses reduced both BKC adsorption and discomfort.
That last word, discomfort, is worth a second glance. A bench study using two types of contact lens on no wearers at all, with no clinical endpoint other than the changes to these physical properties, allows for virtually no conclusion about comfort other than the assertion that it is somehow reduced. That is an important distinction to make, because a phrase such as reduce both discomfort and adsorption is a strong claim to make for a study titled Binding affinity of benzalkonium chloride on contact lens surfaces and the effects on their physical properties.
The ten-to-fifteen minute wait is a convention
Nearly every article advising a wait of ten or fifteen minutes before reinsertion lists no reason for this number at all. There is no clearance curve behind this figure, merely an assumption that has stood for decades, and practitioner literature is no better, citing studies which contradict each other on whether particular package inserts do or do not bear a warning for contact lens wearers. The clinical trials referenced in these journal articles contradict each other on whether extractable BAK from worn lenses was found to clear within the washout period or not, with several finding levels of extractable BAK below the level of laboratory detection after a standard instillation and wear schedule.
That places you in an honest but uncomfortable place, because the caution is warranted but the number behind it is entirely anecdotal. Follow the convention of waiting an arbitrary amount of time and you will spare yourself a potentially uncomfortable period, but nothing in this process is actually known with any precision. There is no other number to follow other than this one. The preservatives found in eye drops, four broad categories of which are listed below, interact with the contact lens in ways that defy generalisation.
Sorting the bottles you will actually see
Rewetting drops are formulated for instillation over lens, but artificial tears and lubricating drops for dry eye are a separate category entirely, and no drop meant for dry eye is entirely safe or entirely unsafe for use over a lens unless stated otherwise on the packaging. Similarly, drops meant to reduce redness and those meant for allergies are a category unto themselves. A preservative-free gel is still a gel and still needs to be removed before sleep, and it is the difference between the label and the optometrist, with neither being wrong but both being limited in what they can say.
The uncomfortable part: do rewetting drops work?
Every consumer article on this topic will tell you that drops with preservatives are generally incompatible with contact lenses, but an evidence-based review published in 2024 and appearing in the Cochrane Database provides much more nuance to this statement. This review analysed seven randomised controlled trials with 463 total participants and evaluated the benefits and harms of using lubricant eye drops for the relief of contact lens discomfort, with the results being that the quality of evidence for all outcomes was categorised as very low (PubMed).
When compared to no treatment for contact lens discomfort, lubricant eye drops may have resulted in a 5.9 point improvement on a 37 point scale from two trials with 119 total participants. When compared to normal saline solution, the improvement may have been no better than drops with no active ingredients, from one trial with 39 participants with a mean difference of 9.5 points on a 0-100 scale with a confidence interval running in both directions.
The review's abstract is explicit about the reasoning behind this conclusion, noting that three of the seven trials carried a high risk of bias, that most trials failed to utilise a valid measure of discomfort, that all were underpowered due to the sheer number of combinations of drops and lenses tested, that follow up periods were short (one to four weeks), that diversity was low (participants were recruited from North America and Europe) and that five of the seven were industry funded.
No trial however reported microbial keratitis, corneal infiltrates or vision threatening events in wearers using these drops with contact lenses, and the review itself suggests that prescribing these drops along with saline solution remains a viable option, with much needed trials currently being lacking.
That is a recommendation, not a statement of fact, and a recommendation to treat yourself to rewetting drops with your lenses in if nothing else, because nothing about lubricant eye drops is known with much precision. A recommendation to use rewetting drops with a label stating that they can be used over a contact lens, and no recommendation at all when using artificial tears for dry eye.
Ayurvedic eye drops and lenses: nobody knows
This final section is one you may not need to read at all, because Ayurvedic eye care is a common domestic practice in India, one that most people would not associate with anything remotely strange or exotic. Rose water is used in tired eyes at the end of a long day, and triphala decoction is washed over the eyes when cool and strained. The Ayurvedic procedure for instilling drops into the eyes is called aschyotana, and is described in detail in the Sushruta Samhita's Uttara Tantra, the Kriyakalpa section, with Vagbhata's Ashtanga Hridaya describing the use of aschyotana for tired, dry, watering, burning and smoky eyes, with drops placed at specific times of day into the open eye from a short distance.
This script has been in use for two millennia, and long predates the existence of the contact lens.
There is no Ayurvedic scripture that prohibits the instillation of drops over a contact lens, but absence of prohibition is not permission, and reading permission into a silence is how accidents happen.
The modern situation is no less clear, because any Ayurvedic proprietary eye drop sold in India is available in multidose drops, and multidose drops by definition contain a preservative of some sort, and those bottles do not carry any lens-compatibility statement, meaning that no Ayurvedic eye drop sold in India has been tested for use with a contact lens in the eye. That is the entire situation in a sentence: the lenses come out, not because the preparation is unsafe, but because no bottle of eye drops, Ayurvedic or otherwise, without a lens-compatibility statement on the label has the right to go in over a contact lens.
Home preparation is an even more precarious matter, because rose water from a kitchen shelf and decoction from the stove are neither sterile nor isotonic nor specially prepared for use with a contact lens and should not be used over one at all.
When to stop reading and call someone
Contact lens wear carries a demonstrable risk of infectious complications, and that risk is the reason why this long and involved discussion exists at all. New onset hazy vision, pain that does not settle, redness that will not pass, photophobia, discharge and decreased visual acuity all require you to remove the lens and see an optometrist the same day, no later, and no drop to be put in your eye should delay you from doing so. Everything else is a conversation, but a conversation with very short branches. If the label permits you to insert the drop with the contact lens in, there is nothing else to discuss. If it does not, you take the lenses out, put the drops in, wait ten to fifteen minutes and put the lenses back in. And if you find yourself needing drops more than a few times a day, or the discomfort is otherwise a regular part of your life, you talk to your optometrist, and not the shelf, about your contact lenses themselves.
