Eye drops have preservatives in them to ensure the sealed product remains sterile until the expiry date
After opening however, the preservative can only ensure the drops are safe for the eye for a period of 28 days. After this, using the drops can cause serious damage to the eye as bacteria may have been introduced.
The ingredients themselves will also not be as effective and could be dangerous. Always record the date eye drops are opened and do not use them after 28 days. Some eye drops now have longer expiry dates after opening - always check the packet.