Emotional poison is forming negative identities within yourself.

情绪毒药是在你自己内心形成消极的身份认同。

Now in my opinion what the author is referring to as emotional poison throughout the book, the negative identities that we form within ourselves when we take things personally.

在我看来，作者在整本书中提到的情绪毒药，就是当我们认定某事与自己有关时，我们内心形成的消极身份认同。