Propofol was a drug that could put a person into an unconscious state that they could not be woken from for a period of time.
When Jack Bauer was re-establishing his cover with Karl Rask, he planned to administer the drug to Kate Morgan so he could deliver her as a hostage but she could not be questioned. Morgan instantly agreed to the plan, injecting herself with the syringe of Propofol, and Jack put her in the trunk of his car. However, Rask had a chemical substance that could counter the effects of the propofol, and had Radko wake her up and torture her for information. ("Day 9: 4:00pm-5:00pm")