My interpretation is opposite of the others. Basically I think this song can be seen as a happy song, as it is a catchy song. Cough syrup has a nasty tasty 99.9% of the time BUT it does help… so after “Waiting for this cough syrup to come down, come down” meaning any problems he may be having or whatever, He realizes that “life is too short ot even care at all, I’m coming up now coming up now out of the blue” meaning he realizes that the problems he had are nothing and life is too short to be stuck on the bad.
I believe that life’s short to get stuck on bad things, I also believe in song he talks about some drug addiction he’s letting behind and love the idea of the zombies (drug dealers and other drug addicted) hunting him. The cough syrup can even mean a medicine to cure him form his addiction. Many ways to understand this lyrics.
Just love this song I discovered on Glee.
So I’ll say it again: Glee is also culture 😉