According to an interview with Shane Briant (Simon) on the 2014 Blu-Ray, real human blood was used in this film. Blood that could no longer be used for transfusions was sourced from the blood bank and used in the film, including in the notorious scene where Victor (Peter Cushing) uses his teeth to clamp the artery of the monster.
According to Shane Briant, he had an extremely hard time finding anything nice to say about The Monster (David Prowse)'s costume. To this day, he calls it "plastic-y". Reportedly, when Prowse asked him what he thought of it, Briant replied: "the feet are fantastic!", upon which Prowse said: "they're mine."