THESE MAY YE EAT. "But not an unclean animal. Has it not already been forbidden by means of a negative commandment? But [this verse is stated] so that [if he eats of it] he transgresses both a positive and a negative commandment." This is Rashi's language, and so it is found in the Torath Kohanim.Vampires who make sure to imbibe only small amounts from any one individual can therefore rest assured that they are fully within the daled amot of halacha. Ghouls, of course, will have to ensure that their victims are fully compensated for the five types of damages that they might incur. The news is still bad for ghouls of the H.P. Lovecraft persuasion who feast on corpses, since they would be in violation of a Torah prohibition of deriving benefit from a dead body.
Now Rabbi Moshe [ben Maimon] said that this verse is in order to forbid human flesh -— "these may ye eat, but not human flesh. Thus the flesh and the milk are forbidden by means of a positive commandment." But we have not found such an interpretation by our Rabbis. Perhaps he [Rabbi Moshe ben Maimon] thought this to be the case because of that which we have learned there in the Torath Kohanim: "I might think that the flesh of those that walk on two legs and the milk of those that walk on two legs should also be forbidden to be eaten by means of a negative commandment? Therefore Scripture says: These ye shall not eat — these are forbidden to be eaten by means of a negative commandment, but the flesh of those that walk on two legs and the milk of those that walk on two legs are not forbidden to be eaten by means of a negative commandment". From this text the Rabbi [Moshe ben Maimon] may have deduced that they are not forbidden by means of a negative commandment, but are forbidden by means of a positive commandment, and he derived it from the verse: these may ye eat.
But the matter is not so. For our Rabbis have clearly said in connection with the blood of those that walk on two legs and the milk of those that walk on two legs that there is not even a commandment to abstain from eating them by Rabbinical enactment. If the flesh thereof would be prohibited [by Scriptural law], then [the blood and milk thereof would also be prohibited in accordance with the general rule]: "anything that comes out of that which is impure, is also impure." The blood of crawling reptiles and that of human beings the Sages have excluded from the prohibition against blood, and they have said: "The blood of the crawling reptile is like its flesh, and one incurs whipping for eating a crawling reptile," meaning that it is not forbidden as blood [for the wanton violation of which one incurs excision]; thus they made it like flesh [but we find no such statement in connection with human blood]. Rather, when they said that there is no negative commandment against eating them, they meant to say that you cannot exclude them on the basis of it [i.e., on the basis of a specific Scriptural verse], and they are thus permitted. According to my opinion, however, this only applies to flesh [or blood] of a live person [which can not be prohibited on the basis of a definitive verse and hence if a person’s teeth are bleeding he may suck the blood thereof and not be afraid of having committed a sin]. However, the Rabbis have learned in connection with a corpse by means of an analogous use of words found when speaking of it and of the heifer whose neck is broken that it is forbidden to have any benefit from it.
(Ramban translation by Charles B. Chavel, which is an anagram for "SHH Clever Cabal". Coincidence? I think not!)