Syringes are useful to stabilize your heart rate. When you "free" any ghost, you can go to the android's room in the main hall (with the moon monument). The android's room is between the Energy Area and the Work Area. In the desk, behind the android, you can find a syringe. When you free another ghost, you can find another syringe. Note: You can see that the freed ghosts are painted in the room's walls.
Echo Night: Beyond