MCU: 15 Majorly Dumb Plot Holes You Never Noticed
4. Why Doesn't Anyone Use Extremis? - Iron Man 3
The MCU has a habit of introducing a plot point with no build-up and then never mentioning it again after it's served its purpose in the story. In Iron Man 3, Stark meets a scientist called Maya Hansen who invented a healing treatment called Extremis that can gives the host regenerative abilities. Not only could Extremis allow a patient to heal from virtually any injury, he or she could regrow limbs within seconds.
We later learn that that Hansen allowed Aldrich Killian to appropriate the Extremis for his own nefarious schemes. After Stark defeats Killian, Extremis is never addressed again and everyone lives happily ever (at least until the next movie).
But Extremis isn't just a new form of treatment - it would change the world. It can rectify paralysis. Quadriplegics would be able to walk again. So how come Extremis' capabilities aren't common knowledge? Can you imagine how annoyed Bucky Barnes would be if he learned he could grow back his lost arm but nobody told him?!