The word poison as I have noticed comes up over and over in Hamlet. It is always used to refer to the old kings death. It may just be me but I believe that the "poison in the ear" line we always see refers to something much more. Perhaps Shakespeare is comparing the poison to words. How words can sometimes enter our ears and poison us with lies. And its these very same lies that make us do crazy things. Just how in Hamlet the character Hamlet is doing all these crazy things because of something a ghost told it. Not only that we also see how deception is a persistent theme in the play. And how is someone deceived? Exactly, through words. When people are deceived they can be lead to believe something that might make them go crazy, ie.Hamlet. And it is this very idea i believe Shakespeare is attempting to portray.