How do you know if you're allergic to penicillin? I mean, do you just take it to find out. I just picked up a prescription for the generic form of Augmentin to prevent an infection from a cat bite. I don't recall being allergic, but I don't recall ever taking penicillin. I'm 35, and I do sort of remember taking a chalky liquid prescription sometime when I was growing up (in the 70s or 80s). Would this have been penicillin? If so, I did not have a reaction.