Blood fasting

Blood fasting is a process that allows vampires to be conjoined mentally, and in part, physically, by experiencing whatever the other feels. The process is brought about when the two vampires drink each other's blood, usual in equal amounts.

The Eighth Doctor once agreed to blood fasting with Joanna Harris, the leader of a group of vampires in San Francisco, despite concerns that it wouldn't be possible for a Time Lord. Their mutual thoughts and feelings meant that both parties could trust each other. (PROSE: Vampire Science)