Can an allergy to tomatoes manifest in diarrhea?

Of course it can. A lot of food allergies can cause G.I disturbances.

Allergic reactions to foods, may have different effects on different people.

Think of milk, or other dairy products. people who are allergic to milk, usually, dont have problems breathing. They get diarrhea.

I am allergic, to melons, and when I would eat them, I would get bad bad chest pain. Who would think that would be an effect of an allergy? right? well, it was.

U should see a doc to make sure, u are allergic to tomatoes, if it has not been diagnosed.

It also could have been that the tomato was not cleaned properly. Or maybe it was old?

My daughter's pediatric allergis would call that a food intolerance, not a true food allergy. True food allergies cause respitory problems, hives, eczema, anaphalaxis. Bloating, gas, diarrhea, etc.. are food intolerances. My daughter has severe food allergies and several food intolerances. If a certain food causes problems for you, then don't eat it. If you're still unsure, schedule a visit w/ an allergist.