The subject is written in a way to try and avoid any spoilers, if you think it should change: feel free.
What does Henry Injects to Adam's neck in order to paralyze him?
In the TV Series Forever S01E22 after Adam shoots Henry with the musket (that theoretically is the one responsible for Henry's immortal state) Henry injects something to Adam, which after, in the hostpital scene, we learn that Adam is now paralyzed.
According to the Creator Matt Miller, Henry injects nothing but air into his brain stem, causing an Air Embolism in his brain, inducing a coma-like state. Not dead, but neutralized. After all, he IS a doctor. A medical revenge seemed to suit Henry best.