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 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.
Nick Fury used tetrodotoxin B to fake his death when he was attacked by Winter Soldier in Steve Rogers' apartment, as a way to stay under HYDRA's radar.
Fury later revealed to Rogers and Natasha Romanoff the use of tetrodotoxin B, as they both had been in presence of what appeared to be Fury's corpse and did not realize he was actually alive.