Gorgon Blood

According to Stheno, gorgon blood from the right side of a gorgon's body can heal someone, although according to Percy it can not heal memories. However, gorgon blood from the left side of a gorgons body will kill someone.

The Son of Neptune
Stheno and Euryale chase Percy. At one point, Stheno reveals to Percy about how gorgon blood can cure or kill, depending on which side of the gorgon's body it came from.

Later, Percy decides to make a deal with Phineas. Both must drink from two bottles of gorgon blood. One contains blood that heals, and the other contains blood that will kill. Percy claims he can not tell either one apart. He relies on Gaea to trick Phineas into drinking the wrong bottle. Phineas disintegrates, whereas Percy just feels a stomach ache that quickly passes. The bottle of gorgon blood apparently did not return Percy's memory.