Demon blood is better than Ovaltine, vitamins, minerals. It makes you big and strong.
"Demon blood" refers to the blood belonging to the human host a demon is currently occupying. Consumption of this substance enhances the drinker's physical and mental capabilities. Consumption on a regular basis can lead to addiction. According to Castiel, a vessel must drink this substance to successfully contain Lucifer, as it greatly strengthens the vessel. Demon blood seems to be somewhat acidic, because when Sam spit the demon blood into one of the hunter's face, it burnt his skin.
Azazel reveals that he fed Sam his blood on the night of his six month birthday, and also reveals that he did so to all the other Special Children. Demon blood either gives or enhances psychic powers.
These powers vary from premonitions to superhuman strength, telekinesis and the touch of death. These power may begin as weak or minor, but if the individual starts to practice, they can improve their powers (eg. Ava & Max Miller.) According to Jake Talley, after submitting to the pull of their powers, the Special Children can perform "all sorts of new Jedi mind tricks". For more on this see: Special Children.
After Azazel dies, Sam's psychic powers seemingly disappear.
When Dean encounters Azazel in 1973, he reveals what he plans to do to Sam:
- Azazel: Cause you know what I'm gonna do to your sibling? I'm gonna stand over their crib and I'm gonna bleed into their mouth.
When Dean tells Sam what Azazel revealed to him, he is shocked to learn that Sam has known for over a year. Later Sam says:
- Sam: I've got demon blood in me, Dean! This disease pumping through my veins, and I can't ever rip it out or scrub it clean! I'm a whole new level of freak! And I'm just trying to take this - this curse... and make something good out of it. Because I have to.
At the end of the episode Sam says he has made a choice, not for Dean or God but for himself, to stop using his powers.
Given that Sam has the ability to exorcise Samhain, Sam must be drinking blood around this time. He must stop soon after.
Through a flashback, Sam reveals that while Dean was in Hell, Ruby taught Sam how to tap into his buried powers, preparing him to take on Lilith. He doesn't mention that part of this included feeding on Ruby's blood.
Ruby alludes to the blood consumption when she tells Sam his abilities are getting flabby:
- Ruby: You know how. You know what you got to do.
- Sam: No, I'm not doing that anymore.
Ruby visits Sam, and Sam again refuses to return to drinking blood (although he doesn't explicitly call it this). However at the end of the episode, he changes his mind and goes to meet with Ruby.
Sam is worried that Dean is emotionally compromised, and will be unable to torture information out of Alastair. After locating Dean with Ruby's spell, Sam begins to doubt his own strength, as Alastair had resisted his previous attempts at exorcism. He drinks enough of Ruby's blood to not only give him the power to exorcise Alastair, but to kill him.
Sam is worried that Chuck Shurley knows about him consuming Ruby's blood and that he will tell Dean, so he confronts him alone. Chuck says he does know about Sam drinking demon blood, but that he didn't write it down because he was afraid it would make Sam "look unsympathetic". He challenges Sam about his motivation for doing drinking the blood, asking whether he does it because it makes him feel in control.
Sam becomes increasingly agitated as the demon blood, presumably from Ruby, he is carrying in a flask runs out, and Ruby won't return his phone calls. When he tries to exorcise a demon with his powers, he fails. When Dean asks what is going on, and says he is scared about what is happening to Sam, Sam says he doesn't know but that he is scared too. When Anna arrives to chastise the Winchesters for losing Jimmy Novak, she comments that Sam seems "different". Later during the fight with the demons, Sam takes the opportunity to cut one of the demons and drink her blood. He uses his powers to exorcise the demon possessing Amelia Novak.
Back on the road, Sam waits for a response from Dean about the revelation of his blood drinking. Dean seems weary and won't talk about it. Sam gets a call from Bobby saying he needs to see them about Apocalypse related business. They get to Bobby's place, and he pretends to show them something in his panic room, but it is a ruse to get Sam inside. Dean and Bobby lock him in.
Withdrawing from the demon blood, Sam hallucinates, has fits, and is flung bodily around Bobby's Panic Room. With no lore to guide them Dean and Bobby are unsure if Sam can survive.
When Sam is released by Castiel he meets up with Ruby and feeds on her blood again. She says his appetite has increased, and that this is a sign he his getting stronger.
Ruby finds another demon for Sam to feed on because she cannot provide sufficient blood for him on her own. Sam is distressed when the demon allows the personality of the frightened woman she is possessing to show through. Sam is confronted with the fact that drinking her blood will kill the woman. Despite this, as he approaches his showdown with Lilith, he agrees to drink from her.
After killing Lilith, Ruby reveals to an appalled Sam that she has been working with Lilith all along to free Lucifer. She tells Sam he didn't need the demon blood, that the choices he has made all year have helped him tap into his own power.
As Lilith's blood forms a spiral on the chapel floor, it frees Lucifer from his imprisonment.
Dean asks Sam if he is "jonesing for another hit of bitch blood". Sam replies "It's weird. Uh, tell you the truth, I'm fine. No shakes, no fever. It's like whoever...put me on that plane cleaned me right up."
When Sam kills two townspeople he thinks are demons with Ruby's knife, he appears drawn to the blood. He tells Dean he wishes he could save people like he used to. Dean retorts - "you mean when you were all hopped up on demon blood?" This leads to Sam's decision to leave Dean, and hunting, because he can't trust himself.
When demons kill a hunter, his friends confront Sam about what one of the demons told them. Sam confesses that he was the one who started the Apocalypse, just as the demon said. Hoping to get revenge for their friend, the hunters ask Sam to drink a vial of demon blood and wipe out all the demons in the town. Sam refuses and he's attacked by Hank, who force-feeds him the blood. Sam spits it back in his face, then beats them up badly, and they leave.
When Famine arrives and awakes in people an irresistible urge for the thing they hunger for most, Sam again starts to crave demon blood. He gets Dean to handcuff him to the bathroom sink, but Famine sends two demons there to tempt him. Sam drinks their blood, and then uses his powers to go and exorcise the other demons Famine brought with him, resisting the urge to drink more blood. When Famine ingests all the demons, Sam turns his powers on the demons within him, disabling him through the demons.
While discussing Sam's plan to say yes to Lucifer and jump in the hole, Castiel reminds him about the demon blood. To take in Lucifer, Castiel says, would be more than Sam has ever drunk. He says the blood "strengthens the vessels, keeps it from exploding," and notes that Nick, Lucifer's current vessel, is drinking "gallons" of the stuff. It is speculated that this may have been Ruby's intent all along in getting Sam addicted to demon blood - strengthening him to become Lucifer's vessel.
Dean, Castiel, and Sam bleed a group of demons and store their blood. Sam drinks it in Detroit as he prepares to confront Lucifer.
Bobby Singer realizes that the Blood of The Ruler of Fallen Humanity they need for the Leviathan-killing weapon is the blood of the demon Crowley. In order to get his blood, Sam and Dean summon him and Crowley agrees to give them his blood for the weapon last as he is worried about what his enemies will do if they get their hands on a vial of his blood. Later, Leviathan leader Dick Roman summons Crowley into a Devil's Trap in order to prevent the Winchesters from getting his blood.
Leviathan leader Dick Roman summons the demon Crowley into a Devil's Trap in order to make a deal with him to prevent him from giving his blood to the Winchesters. In exchange for the Leviathans giving Canada to the demons, Dick asks Crowley to give the blood of a minor demon from New Jersey to Sam and Dean and pass it off as his own, not killing him right off the bat as he figures Crowley has a vial of his blood stashed away in case that happens to be sent to Sam and Dean. Crowley agrees and they work out an official deal. However, Crowley goes back on the deal and informs the Winchesters about it and gives them the right blood. Using Crowley's blood as part of the weapon, Dean is able to kill Dick and effectively defeat the Leviathans.