Any drug can cause an allergic reaction. It doesn't matter if you've taken a drug in the past and not had a reaction to it; allergic reactions don't always occur on the first exposure to a drug. What people refer to as "allergies" to drugs are often side effects, not a reaction of the immune system that produces antibodies and histamine, the Merck Manual states. Allergic reactions range from mild to life threatening anaphylaxis.
Skin Reactions
Skin reactions are common responses to a drug allergy. A skin reaction can take many different forms; some drug allergies cause a rash; others cause hives. Hives are reddened welts that can occur anywhere on the body and typically move from place to place. The arms, legs and trunk are the most common sites for hives, which can last several hours to several days. Angioedema, swelling around the eyes, mouth and lips can be a sign of an allergic drug reaction. Skin may be itchy.
Skin reactions can occur immediately after a drug is given, especially if it's injected or given through an intravenous infusion, or may not start until you've been taking the drug for several days, the Mayo Clinic states. Antihistamines and corticosteroids may be given to treat skin reactions; epinephrine may be required if facial swelling occurs.
Respiratory Reactions
Respiratory reactions are also common in drug allergies. Shortness of breath, wheezing, an itching feeling in the throat, tightness in the chest or a feeling of pressure may all occur in allergic drug reactions.
Anaphylaxis
Anaphylaxis is a severe allergic reaction to a drug and can in rare instances be fatal. Anaphylaxis can occur within minutes of taking a drug; symptoms include constriction of the throat that may completely occlude the airway, extremely low blood pressure, lightheadedness, dizziness and confusion. Anaphylaxis may cause gastrointestinal effects such as nausea, vomiting and diarrhea. Swelling of the face, including the eyes, tongue and lips may occur, and the heartbeat may be fast, irregular and weak.
A person in anaphylactic shock can quickly go into a coma or stop breathing. Anaphylaxis is a medical emergency, according to the Mayo Clinic. Epinephrine injection is necessary to reverse the effects of anaphylactic shock.
