Yes, this has happened on numerous occasions with me, and it is what happened with the Benedryl. I wasn't always allergic to it. It was only after I began taking it on a regular basis after I became severe that I gradually developed an anaphylactic reaction to it. It's happened with foods, it's happene with supplements, and it happened with wool. All things that were fine at one time for me, but my body developed an allergic reaction to. Definitely possible.