On rare occasions, ketchup can cause an allergic reaction. Generally, this is connected to eating too much ketchup, but very few people are affected by this allergy. An allergy to ketchup is actually an allergy to tomatoes. Symptoms of tomato allergy can include nausea, vomiting, skin rash, and swelling of the face, mouth, tongue, and throat.
Again, an allergic reaction to ketchup is rare! If you are in any doubt, please consult a medical professional. It is possible to be tested for allergies if you think you have one.