Adverse reactions to drugs

Untoward reactions to medications are common.  They range from minor side effects that can be tolerated to serious and fatal reactions.  These reactions can be divided into side effects, toxic reactions, allergic reactions and idiosyncrasy.

Side effects are unwanted effects that are caused by the normal mechanisms of actions of the drug.  Examples are sleeplessness from decongestant drugs such as pseudoephedrine, and fluid retention and weight gain from prednisone.

Toxic reactions occur when the patient is overdosed with the drug or when there is increased sensitivity to usual doses.  This can occur with theophylline, when it causes vomiting or seizures.

Idiosyncratic reactions are unexpected reactions that do not appear to have an immune mechanism.  An example of an idiosyncratic reaction is sensitivity to aspirin causing an attack of asthma.

Allergic reactions are unexpected reactions that are not caused by the normal action of the drug and are due to stimulation of the immune system by the drug.  The immune system may react in a variety of ways, most commonly by causing proliferation of lymphocytes which form antibodies.  Antibodies may cause reactions when they combine with the drug. 

Side effects and toxic reactions can occur in any individual, but idiosyncratic reactions and allergic reactions occur only in certain susceptible individuals. 

How do we recognize an allergic reaction?
Allergic reactions do not occur with the first exposure to the drug because it takes time for the immune system to become sensitized to it by proliferating drug-specific lymphocytes.  Allergic reactions that begin almost immediately after a dose include hives (urticaria), angioedema and anaphylaxis.  There may be a variety of other reactions including skin rashes other than urticaria, and problems caused by damage to formed elements of the blood.  These reactions occur hours or days after the drug is taken.

Which drugs cause allergy?
Most, but not all drugs have a risk of causing allergic reactions.  Antibiotics, particularly penicillins, cephalosporins and sulfa drugs are the most common offenders.

Testing for drug allergy
Very few drugs can be tested reliably.  Skin testing may be done with some of the penicillins, cephalosporins, certain vaccines and local anesthetics.  Blood testing for drug allergy has not been sufficiently well validated by scientific research to justify its use in the clinic.  When no testing is available and suspicion of drug allergy is low, the drug may be administered cautiously by test dosing under close supervision.