I believe that the "one dose" cure was tailor made specifically by Questiyah for Silas only. It will not work on the other supernaturals (Klaus and the vampires) if one of them takes it. Either there will be serious repercussions such as instant death, or nothing will happen at all. Silas was supposed to take it himself but he decided to lay holding it for 2000 years until he was released from his so called coffin and seek revenge. That is why he wants Bonnie to commit the final sacrifice of killing twelve so he can have full powers to do what he wants. What do you think?