Plasmids are special serums made from processed ADAM, that introduce modified stem cells into the body, allowing for genetic modification and mutation, giving the user what some might call "super powers". Their use requires a supply of EVE. Plasmid bottles are recognizable by their deep red color.
Plasmids are found, obtained, or bought with ADAM throughout the course of BioShock. Jack has a limited number of Plasmid Slots for actively equipped Plasmids, and his currently equipped Plasmids can be changed at a Gene Bank. The player may purchase additional slots and select Plasmids at a Gatherer's Garden. The player can purchase the third level versions of Plasmids without having the first two levels as a prerequisite.
A file containing names of Plasmids and tonics removed during the development of BioShock can be found in the BioShock game files under the name Predefined_Plasmids.txt.
The name of the Plasmid used by Tenenbaum to cure Little Sisters is "Little Sister Antidote," according to the localization files of BioShock 2 which featured the same Plasmid given directly to Subject Delta in previous versions of the game.