Currently the Alchemist has no means to defensively drink his extracts, mutagens or anything else.
Can an Alchemist cast defensively or like drinking potions, he can't do that?
The downside is no, they can't throw or drink defensively.
The GOOD thing is that since their bombs aren't spells, grappling them is... pretty much asking to have a full attacks worth of bombs thrown into your face at point blank range.
And they can cast in heavy armor without arcane spell failure chance.