Blood tablets are a type of substitute for blood, consumed by dropping one to two tablets into a glass of water turning the water blood-colored, they are also often ingested directly depending on one's preference.
Some ex-humans cannot accept blood tablets as their bodies physically reject them. Zero was included in this, until the second arc after he completed his fragment, he was capable of eating them.
It is usually only required to take one to two tablets at a time. The exceptions include a young Kaname, following the death of his parents, Ruka and Kain noted the alarming amount of blood tablets that Kaname swallows. Kaien Cross finds Zero consuming a high amount of tablets due to his high thirst and inability to quench it through tablets and hunting.
Sara Shirabuki has been involved with the pharmaceutical branch of the Ichijo company that Ichijo used to run and has had highly tasty blood tablets developed. Rima Toya expressed concern over the fact that the new tablets had been rushed out to the market and not had time to be fully tested.
The tablets are revealed to contain Pureblood blood to make them more tasty. Sara confirms that her blood is used in the tablets. However, by using the blood of a Pureblood, it allows Sara to take control over the vampires who consume the tablets. It also means that these highly addictive tablets can force an aggressive response should they be taken away, apparently without the vampires being fully aware of themselves. This is shown when Yuki is attacked and bitten by two vampires after taking their tablets away, only to find out that drinking her blood reversed the effects and turned them back to normal.
After realising that Yuki's blood acts as a countermeasure against Sara's new tablets, she sets about taking all of the tablets given out from the students. This is not an easy feat, as students act out violently and attempt to chase and attack Yuki and the other students. Yuki is attacked once more and comments that it hurts her. In a conversation with Aido, who is revealed to have a vial of her blood to hand, she demands that everyone should drink her blood. However, Aido points out that Yuki could be completely drained of her blood. It is then revealed to Yuki that the department of pharmaceuticals in Ichijo's company are the ones producing Sara's tablets, and Yuki agrees with Aido that the department must be stopped.
Later, Aido meets with the distraught Night Class students who have been cured by Yuki's blood. He proposes that they use Yuki's blood to create new tablets and asks for their help in producing them and removing Sara's from the marketplace. Aido promises Yuki that he will destroy those tablets using the blood she provided.