What is "Pluto" and why don't we have access to it?

[PArcher]

Infact you could take it further and have the head canon that the CD on potions is representing it not being safe to take higher dosages of the potion in such a short amount of time.

I'm pretty sure a very early quest states just that; you can't take any potion soon after another or it has some really detrimental effect on you.