lol I wonder if it's somewhat aphrodisiac?? Not as an individual thing, but as a vampire characteristic from Mistress, it'd make a lot of sense.
As a blood sucking vampire, having some type of neurologic(or aphrodisiac?) "poison" injected to make the victim actively WANT to give their blood to you would be marvelous and well thought-out, but I bet it's nothing more than just "It's a GL manga and they're the main couple" typathing. Excited for the next chapters nonetheless though :]