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 Ichijo pharmaceutical company that Ichijo used to run and has had high 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.