The anesthetics mostly expire in about half an hour after the eye is culled from the iron slag. Eyes into the iron slag for culling before, often is required to the conjunctival sac with surface anesthetic drops, such as bupivacaine eye drops, mainly used to anesthetize the surface of the eye conjunctiva and cornea of the nerve endings, about half an hour or so to expire. However, everyone metabolizes drugs differently, so the exact time may vary somewhat. As the anesthetic wears off, discomfort such as foreign body sensation and tearing may reappear. If it is necessary to continue to remove the foreign body of iron slag, you can once again use the drops into the conjunctival sac, and most of them will provide immediate pain relief. However, surface anesthetics are not clinically recommended as a therapeutic agent, and damage to the corneal conjunctiva may occur with multiple drops.