Prolific Inventor

From BioShock Wiki

Prolific Inventor is an Engineering Tonic which causes the user to receive twice as many items as normal when inventing at a U-Invent machine.

[edit] Game Caption

Inventing is already a valuable pastime in the resource-strapped halls of Rapture. Make yourself a Prolific Inventor and you'll be the envy of your peers as you turn worthless junk into valuable commodities.

[edit] Location

This tonic can only be acquired from the second-to-last gift given by the Little Sisters for rescuing them.