Probably the key prop of the film are pens like this, containing some kind of drug used throrough the movie by all the movie characters to prove they're not "infected". Zeke (Josh Hartnett) deals also with these drugs at the beginning of the movie, only to find out later they have a nice weapon in their hands...

COA: StarWares

Images 2 & 3: Several moments in the film where you can see these pens...