Side effects are the secondary and undesired effects of a drug. These are expected reactions that occur with the administration of a drug, and every patient receiving that drug is at risk for experiencing side effects. Examples of side effects include drowsiness with the administration of diphenhydramine and nausea with the administration of chemotherapeutic drugs.
Allergic reactions are aberrant immune responses to an antigen or an allergen. An antigen is defined as a toxin or substance that incites an allergic reaction in the body, especially one that leads to the production of antibodies. An allergen is defined as a substance that incites an allergic reaction.
It has been previously reported that true hypersensitivity reactions to drugs probably account for less than 10% of all adverse reactions to drugs.1 In general, most drugs are small organic molecules which are incapable of stimulating a full immune response independently. However, when these drug molecules or their metabolites become bound to proteins, they form drug-protein complexes which then act as allergens and stimulate an immune system response. The ability of a drug or a metabolite to sensitize the immune system depends on its ability to bind tissue proteins and become an allergen.2
The term allergy implies an immediate hypersensitivity reaction mediated by immunoglobulin E (IgE) antibody. However, many allergic reactions are not IgE mediated. The subsequent reaction produces a wide range of clinical symptoms ranging from itching to anaphylaxis. Anaphylaxis is the most severe form of an allergic reaction, and it can occur suddenly and may, eventually, lead to death.
Anaphylaxis occurs when the body’s immune system produces a substance called immunoglobin E. This substance binds to cells in the body called mast cells that then release chemicals that cause an allergic reaction, such as redness, swelling, or hives. After this process occurs once, it is likely to get more severe with each additional exposure.5
The anaphylactic reaction may occur within a few seconds to a few minutes. Allergic reactions after the administration of drugs orally tend to occur at unpredictable times. Anaphylactic reactions currently account for 1 out of every 2000 ambulance requests and are reported to be fatal in as many as 2% of all the cases. Some epidemiologic studies have shown that up to 5,100 hospitalizations have been linked to anaphylactic reactions.3