Tetrodotoxin B is a drug made by Bruce Banner to slow down his own heart rate to about one beat per minute to keep him from transforming into the Hulk.
Bruce Banner developed Tetrodotoxin B to lower his heart rate, in order to keep from transforming into Hulk. However, its use was not as effective as he intended.
Fury later revealed to Rogers and Natasha Romanoff the use of the Tetrodotoxin B, as they both had been in presence of what appeared to be Fury's corpse and did not manage to realize he was actually alive.
- Tetrodotoxin is a real neurotoxin and is often referenced in films.