Antibiotics are used to cure infections. When the temperature of a player drops below 36C, when spending any time around an already infected player, or being attacked by a zombie, there is a chance of getting an infection. Infections cause the player to slowly lose blood, down to a lower limit of 6000. Antibiotics are uncommon, and only found in hospitals, medical tents, and helicopter crash sites. Antibiotics take one slot of your inventory or bag pack.
This page contains information about the mod version of DayZ. Standalone information is available here.