How To Treat Allergic Reaction Caused By Levothyroxine?

I just recently found out im having an allergic reaction to Levothyroxine, probaly the yellow dye in it. I took one this morning before knowing this was the problem to the symptoms i have been feeling. Is it o.k. to take a Benadryl allergy pill until it gets out of my system, and how long will it take to leave my system?