Plasmids and gene tonics appear to both come in a bottle and function the same way. Is there a difference between the two?
Active Plasmids require EVE for use, while passive Plasmids, called Gene Tonics, provide an effect merely by being equipped. 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.
While active Plasmids require EVE for use, Tonics provide an effect merely by being equipped.
In BioShock, Jack is able to choose from a vast selection of Tonics to equip. With Tonic Slot upgrades, the player can equip up to eighteen Tonics, which can be swapped at a Gene Bank. Tonics have three classifications, Combat, Engineering and Physical Tonics and only six Tonics per classification may be equipped at one time. Tonics are found in Rapture or purchased at Gatherer's Gardens.
A Combat, Engineering, and a Physical Tonic bottle, respectively:
Plasmids are active skills that you can equip. You must use them, and each use costs a certain amount of EVE.
Gene Tonics are passive skills. You can't use/cast them, they are always active as long as they are equipped. They tend to boost your stats, make hacking easier, or a number of other things.