Description from original post (created 2012-05-19 02:02):
Extremis was the name given to a "robotic microsurgery" experimental drug created at Futurepharm Corporation in Texas, for the US military.
A project created the nanotechnology serum,as an attempt to recreate the Super-Soldier Serum, which interfaces with the brain's 'repair center' and directs the body to reconstruct as if it were damaged and needed total repair.Before the subject is injected with the serum, The body's redesign is programmable to the point where superhuman abilities can be developed, making it a biotech weapon and highly beneficial military property.
In an attempt to regain the stolen serum, Stark as Iron Man is injured battling a domestic terrorist, Mallen, who had been altered by the Extremis drug into a superhuman killing machine. 
Added by Rcn
Seeing no other option, he took a modified dose of Extremis to heal his injures and improve his neural interface with the armor and other technologies. The revitalized Iron Man then set out after Mallen. E From that point on Tony utilized the Extremis in each of his armor designs and became accustomed to the power and convenience it provided.
During Marvel Secert Invasion, The invading skrull queen Veranke infected Tony's body with a virus that shutdown Extremis and the rest of stark tech. Tony believed the Extremis had been permanently rendered useless by the virus.
It wasn't until Tony deleted the contents of his brain, and installed an older version of his memories along with a new arc reactor, that he regained control and use of the Extremis. Using his newly designed suit titled "Bleeding Edge Armor", along with the energy supplied by his new arc reactor, Tony was able to utilize the Extremis to a greater extent.