We really do not know WHAT it was that Kristen injected. It was not said, and so far, has not been said. EJ lost a great deal of blood, was shot in the back, and it was hours and hours, maybe even days, before he was found. In Will's case, he quite possibly was not dead yet, though thought to be.
And Heather is right. Been 4 years. Put it all to rest already. Every time Ali sweeney makes a guest appearance, Sami is convinced EJ is alive somewhere. Really? But he doesn't try to contact her, get her to come wherever he is? She needs a shrink, or a good writer to think up something else for her. This is old, is boring, and Heather is right. If Sami had remained a regular in Salem, she would already be with someone else, who would be her heart's desire, and she would be kicking herself for her involvement with the man who subjected her to all those horrible things.