Yes the serum does exist, but what do you mean? Although Dan does drink the serum at the end, in the preview of Day of Doom, it is revealed that Amy poured the serum in the drain and replaced it with some sort of vegetable mix.
No, the preview is only on the kindle version of Trust No One.
Amy put it in the drain without Dan knowing. That means that our last hope could be a fish...
What do you think?