Sam's powers become an important part of the fourth season as they are shown to fluctuate – at times so weak that he can barely exorcise a demon, whereas at other times they are strong enough to kill a demon. It is revealed that drinking demon blood makes Sam's powers grow stronger while at the same time making him "cold" and "arrogant". Sam eventually becomes addicted to demon blood, a fact that Dean soon discovers. With Bobby's help they lock Sam in a protected panic room so that he can detox, but the withdrawal symptoms are painful and he begins experiencing hallucinations. Just as the process becomes easier for Sam, Castiel appears and frees Sam. He meets up with Ruby and learns that he will need to drink more demon blood than he has ever done in order to have the strength to kill Lilith. Dean finds and confronts them and tries to kill Ruby, but Sam intervenes. He and Dean argue about how to stop the Apocalypse, whereupon Dean tells Sam he's changing into a monster, leading to a vicious fight between the brothers. Sam is the victor and as he leaves, Dean tells him not to return – just as their father John, years ago, told Sam not to return when he left for college.