Eye drops are effective at any time within their expiration date; once opened, they are ineffective regardless of whether they are opened or not. The shelf life of eye drops after opening is generally one month. Why don't most eye drops specify this period? Because this "time" is closely related to the season, the storage environment, and the patient's usage habits (such as whether the drops come into contact with the eyes), making it impossible to determine precisely. Unsealed eye drops generally have a shelf life of 1-3 years, as detailed in the product instructions. Once opened, the shelf life is generally one month, and patients should use them within that one-month period. Otherwise, expired drops will not only be ineffective but may also easily cause allergic and toxic reactions.
So how many days should the usage period be? Previously, many ophthalmic preparations provided almost no detailed explanation on this. Nowadays, if you carefully read the instructions for each ophthalmic preparation before use, you will almost always find this precaution in the densely written, fly-head-sized text: "Ophthalmic preparations can be used for a maximum of 4 weeks after opening."
The reasoning behind the rule of "maximum use for 4 weeks" is not complicated. Among the many precautions in the instructions for use of ophthalmic preparations, there is actually one: "Do not let the bottle tip touch your eyes when applying eye drops, and tighten the cap after use to avoid contaminating the medicine." Experts explain that once ophthalmic preparations are opened, they are very easy to be contaminated by tears and microorganisms in the air during use and storage, thus creating potential safety hazards.