Did John Walker Injected the Serum?

John Walker injected himself with the last remaining vial of Nagel's serum shortly after being given the mantle of Captain America. As the serum enhances one's inner traits, it exacerbated Walker's mental instability, causing him to become significantly more violent and cruel.

Did John take the Super Soldier Serum?

The new Captain America, John Walker, has now also taken the super soldier serum in the hopes of being as strong as Steve Rogers - and has proven that it takes more than just physical strength to be Captain America.

When did John Walker inject the serum?

After getting frustrated at how easily Bucky stopped him and having convinced himself the only thing holding him back was not being a Super Soldier, in episode 4, Walker (Wyatt Russell) gave in to temptation and took the Power Broker's serum.