Dead man's blood is a substance that is like poison to vampires, which is blood from dead people. This is an essential tool when hunting vampires, as it can create enough pain to incapacitate a vampire and in some cases, knock them unconscious.
Some vampires are rendered unconscious by dead man's blood with it acting like a sedative to them. This appears to depend on the amount of dead man's blood they are given and possibly the vampire's age.
Although, dead man's blood does wear off, and the vampire will be able to regain his or her strength. So far, there is only one known vampire able to still move after being injected with it: the Alpha Vampire, their father, the strongest and oldest vampire in existence.
When Celia was injected with dead man's blood, the veins on the right side of her face become prominent and her right eye began to become bloodshot. She was also not rendered unconscious, but was instead effectively paralyzed. After her son was injected directly in the heart, he could be seen lying on the floor with slightly less prominent veins in his face, noticeably twitching and with his eyes open but otherwise unable to move.
When a person is turned into a vampire, injected with dead man's blood and then cured, they appear to suffer no ill effects from the dead man's blood afterwards. When Doug Stover regained consciousness after being injected with dead man's blood and then cured, he showed no ill effects from the dead man's blood that he had been injected with.
While hunting Luther's nest, John Winchester sends Dean to get dead man's blood from the local morgue. Once they have it, the Winchesters ambush Hank and Kate with crossbows loaded with arrows dipped in dead man's blood. After using the arrows to take down the two vampires, John kills Hank and kidnaps Kate.
Later, while John is negotiating with Luther, Kate recovers from the dead man's blood, breaks free and attacks John. Before the vampires can kill John, Dean takes down two with dead man's blood soaked crossbow arrows.
After Dean gets turned into a vampire by Boris, Samuel Campbell gives Dean a syringe full of enough dead man's blood to "drop a linebacker and then some." When Dean attempts to inject Boris with the syringe, a drop of the blood falls to the floor, catching Boris' attention. During the fight that follows with Boris's nest, Dean uses the dead man's blood to subdue one of the attacking vampires.
When the Alpha Vampire is captured, he is hooked up to gallons of dead man's blood to dull his powers. Though the dead man's blood prevents the Alpha Vampire from summoning more vampires for help, it does not weaken him completely and he is eventually able to break free. Samuel produces two syringes of dead man's blood, all they have left, as one of the weapons they can use to fight the Alpha Vampire. As the Alpha Vampire attacks Sam, the demon possessing Christian Campbell injects the Alpha Vampire in the back of the neck with the dead man's blood, allowing the demon and two others to capture him.
When he teams up with Benny Lafitte to take down Desmond, Dean arms himself with a syringe of dead man's blood from the trunk of the Impala. During the fight that follows, Desmond disarms Dean and destroys the syringe, but Desmond is killed by Benny before he can harm Dean.
As they hunt the vampires that supposedly killed their families, Krissy's group arms themselves with small tranquilizer guns loaded with dead man's blood. Krissy uses these darts to take down Jimmy Day before he is killed by Josephine Barnes. After Victor Rogers is revealed to be working with Seth, Seth takes Aidan hostage in an attempt to escape. As Josephine distracts Victor, Krissy shoots Seth in the right eye and chest with darts loaded with dead man's blood, taking him down.
After Jody Mills kills Cody and calls the Winchesters in for help, they prepare to go after Celia's Family. Before leaving, Sam asks if Jody has heard of dead man's blood and he explains that its like "a horse tranq" for vampires. Jody agrees that she could use some in case the vampires come after her while the Winchesters are gone.
As the Winchesters prepare to raid the nest to rescue Alex and kill the vampires, they and Jody arm themselves with syringes of dead man's blood as well as machetes. After the group is captured, the apparently unconscious Dean suddenly awakens and injects one of the vampires in the heart with dead man's blood, knocking him out and allowing Dean to kill Connor. In the basement, Celia goes through Jody's weapons, including her dead man's blood, before moving to kill Jody. In order to save Jody, Alex injects Celia from behind with a syringe of dead man's blood, rendering her paralyzed. Jody then kills Celia.
While holding Sam captive, Lady Toni Bevell explains the British Men of Letters system for dealing with monsters in England. As she speaks, a vampire is seen being caught by British Men of Letters operatives in London, injected with dead man's blood and brought before Arthur Ketch for execution.
After Doug Stover is turned into a vampire by Marlon, Dean injects Doug with dead man's blood to keep Doug from harming Donna Hanscum. Dean reassures Donna that Doug will be alright and that he has sedated Doug with dead man's blood until they can get Marlon's blood to create the vampire cure for Doug. After Doug regains consciousness again as a human, he shows no ill effects from the dead man's blood.