In The Half Blood Prince, Professor Slughorn froze up when
Ron got poisoned from the whiskey after he was cured from the love potion
If Slughorn is as good as people say he is and "always keeps a cure for Veritaserum" on his person, why would he then freeze up as if to not know what to do when a student would get poisoned?
And why wouldn't he immediately think to
use a bezoar to cure Ron if a bezoar is known to cure almost all poisons?