Drug Allergies

Reactions to drugs are estimated to occur in 1 to 15% of medication courses.  However, most of these reactions are mild and self-limited.  Only eliminates 5 to 10% of these reactions have a true allergic basis.

Non-allergic drug reactions include toxic over dosage, toxic side-effects (such as drowsiness from antihistamines), intolerance, and drug interactions (action of a drug in combination with another drug).

Allergic reactions to drugs occur because previous or continuous exposure has caused development of allergy antibodies or sensitized T-lymphocytes.  Usually the diagnosis is based on clinical observation.  These reactions can occur immediately, within hours, or be delayed for several days.

There are four basic immunologic mechanisms that can cause drug allergies:

1)  Immediate IgE reactions are manifested by itching, hives, angioedema (swelling), difficulty breathing, drop in blood pressure, vomiting, and diarrhea.  Penicillin and it's derivatives are the most common cause of this immediate, anaphylactic reaction.  Other causes include other antibiotics, insulin, NSA10S, and vaccines.  Immediate reactions can also occur with radiocontrast media used for X-ray studies; however, this is not an IgE mediated reaction. 

2)   Many drugs can cause a cytotoxic reaction where red blood cells, white blood cells, and platelets are affected.

3)  Serum sickness is characterized by fever, hives, rashes, tissue swelling, lymph node swelling, joint swelling, and inflammation of the kidneys and liver can also occur.  These reactions typically occur 1 to 4 weeks after drug exposure.

4)  Contact dermatitis can occur 1 to 3 days after exposure to topical medications such as local anesthetics, antibiotics, and preservatives in topical drugs.

Penicillins are the most common causes of allergic drug reactions and can occur in approximately 2% of courses of penicillin therapy.  Up to 10% of these reactions are life-threatening.  Most of these reactions occur between the ages of 20 and 49.  Skin testing is available to detect patients at risk for severe acute allergic reactions to penicillin.

Cephalosporins are another large group of antibiotics and about 8% of patients with penicillin allergy will develop reactions to cephalosporins.  There is no accurate test to detect cephalosporin allergy.

Sulpha drugs have a high incidence of allergic reactions.  Two to ten percent will develop rashes and hives to these drugs.

Aspirin and nonsteroidal anti-inflammatory drugs (e.g. ibuprofen) can cause adverse reactions including bronchospasm with difficulty breathing and generalized hives.  This is much more common in patients with history as asthma and nasal polyps.

Tests to evaluate drug allergies are limited; therefore the diagnosis usually made by clinical history and physical findings.  Open challenges under direction of a trained allergist can be performed if the probability of drug allergy is low.  Desensitization may be used to administer a drug when the probability of a severe reaction is high.  Pretreatment with corticosteroids and antihistamines can reduce the risk of a severe reaction when certain drugs, such as with radiocontrast media for X-ray studies is required.