Extremis is a bio-enhancer that modifies biology and gives the user selective range of superhuman abilities. Every version of Extremis affects the person who takes it differently, granting different abilities. When the user takes the serum, the user vomits a black and red chrysalis that then encompases him/her. It then morphs the user into its new form and then emerges with new superhuman abilities.
One of the versions of it changed Mallen into a biological combat machine, but also drove him even more insane than he already was. Tony took only a drop of it which repaired his heart, enhanced his biology, and gave him the ability to mentally communicate with computers and all forms of technology, specifically his armor.
In the future, it is unknown what the Extremis is used in the future but it used by everyone in 2099, but in Version 16.5, which is said by Andros Stark (Iron Man 2099).
Stark injected himself with a small drop of Extremis. When he awoke from its black chrysalis, his natural healing was enhanced, which was able to completely heal his damaged heart. His biology was enhanced, giving him increased strength and increased reaction time. It also increases his intellect and mental computation abilities.
It has also given him the ability to control all types of digital technology with his mind. Its range of influence goes from all over the world to orbital distances, like satellites. He can also sense data inside computers. This power also gives him a bio-cybernetic link to his armor, controlling it even from a distance and allows him to control its functions faster and more efficiently.
Mallen stole the Extremis serum and injected himself with it. He was encased in a black chrysalis, and when he broke out of it, he was given superhuman abilities and a mutated human look.
Extremis gave him superhuman strength, speed, regeneration, endurance, stamina, agility and durability. He also gained the ability to generate intense bolts of electricity and fire generation. As a side-effect, it gives him occasional hallucinations.
Andros Stark, the Iron Man of 2099, uses an improved version of Extremis called "Extremis 16.5" as his OS. It allows him an enhanced biology as well as allows him to control his suit's functions more efficiently, just like the present day Iron Man's Extremis. He remarked that everyone uses it in 2099.
- In Rage of the Hulk, Pepper Potts thinks that when he does Extremis on his armor in the Armory, it is "super-creepy".
- Tony cannot control technology that is too alien to understand as seen during the Makluan Invasion episodes when he couldn't manipulate the aerial attackers or the mothership.