Don't use old drops
You can contaminate a bottle of eye
drops by touching the eye, but the drops inside have preservative to protect them, and the bacteria
on the outside probably die by drying out.
Multi-dose bottles have preservatives; single use vials do not. In a fridge, the longest you should probably keep the drops is 30 days, once opened. Link: http://drkisling.Com/2011/09/19/how-long-can-you-use-eye-drops/.