In episode 17 of Monogatari Second Season (the first episode of Onimonogatari), when Koyomi and Mayoi see the strange "darkness", an image of the structure of a chemical compound is displayed, repeatedly:
Shortly afterwards, a number of different chemical compounds are displayed (numbers not present in original):
What are these compounds?
The first image is the structural formula of epinephrine (or adrenaline, depending on where you live).
Epinephrine is a neurotransmitter that plays a significant role in the fight-or-flight response, and so its appearance here makes sense - Koyomi is certainly about to take flight upon seeing the "darkness".
The second image contains the following compounds:
All of these are neurotransmitters, as well, though none of them are as localized to the fight-or-flight response as epinephrine is.