This is the type of allergic reaction that occurs when a person takes certain types of medications that the body mistakenly believes is a foreign substance that needs to be expelled from the body. In its fight to remove the substance, the body begins producing huge amounts of the chemical called histamine. When this happens the individual can begin to experience symptoms such as dizziness, anxiety, fainting, hives, skin rash, swelling of the throat and mouth, congestion, and eyes and skin that feels itchy. In some cases the allergic reaction that people experience can be extremely serious and can cause such problems as confusion, breathing difficulties, abdominal problems, anxiety, diarrhea, fainting, nausea, and a rapid pulse. Although there is a variety of different over-the-counter and prescription forms of medicines that people develop an allergy to, one of the most well known medications for causing allergic reactions would be penicillin. Others would include antibiotics that have a similarity to penicillin, anticonvulsants, sulfa medications, iodine used as a contrast dye for x-rays, barbiturates, and insulin.