I've got this too. Doctors said it was my OCD as I have contamination fears about poisons and drugs, especially on my food. After I eat, even if its something I've ate a million times in the past, I sit in panic for 5 mins to see if I take a severe reaction and when I don't, I still sit in panic for a few hours in case I have been poisoned in some way even though I know I haven't. Its just an intrusive thought I can't get rid of.