Saliva from vampire bats was being used to help treat stroke victims as part of a trial at UK hospitals, The (London) Sunday Telegraph reported.
A drug treatment derived from the substance can thin blood and help to break up clots on the brain, research showed.
Current medicines that thin the blood need to be administered within four hours of a stroke to be effective, but the new drug can have a similar effect up to nine hours later.
Bat saliva was chosen because protein found in the substance keeps the blood of their prey thin enough to drink.
The trial is currently taking place in 40 hospitals across the UK, with 400 patients receiving the new drug.
Doctors said that if the trials are successful, the treatment could become widespread within three years.