Before the service in the ship's chapel, McCoy tells Spock that Theragen might be an antidote for the interspace distortion effect on the crew. Spock shows no reaction. Later in Spock's cabin, McCoy tells Spock and Scotty that Theragen is the antidote. This time Spock is obviously surprised and reacts by saying "Theragan! A nerve gas used by the Klingons!" So, Spock, did you just tune McCoy out the first time he mentioned Theragen?