thera tears eye drops

Once I open a vial of single use peservative free/sterile eye drops, how long are they safe to use?

I use “Thera Tears” lubricate eye drops in single use vials. However, each vial will provide about 15 drops of which I only add to each eye a couple drops per day. They have a resealable plastic push on cap which is tight, but maybe not 100% air tight. Is is safe to use this vial 3-4 days?

I wouldn’t go 3-4 days – maybe 1 day. The reason those vials are intended for single use is that they don’t contain preservatives that kill bacteria. You definitely don’t want anything to start growing in there and then put it in your eye.