Allergic drug reactions are those that are caused by an immune mechanism. Examples include itchy rashes, hives, swelling, coughing, wheezing and difficulty breathing. In contrast, non-allergic or intolerance type reactions are unrelated to the immune system and are usually a type of side effect. Examples include diarrhea or headache. Allergic drug reactions occur only rarely and only in predisposed individuals. On the other hand, non-allergic reactions occur in a much larger proportion of patients taking a medication.
Drug desensitization, also called induction of drug tolerance, is a method of safely administering a medication to a patient who is allergic to it. It should be performed by an allergist / immunologist and in an appropriate setting to treat possible reactions. This procedure temporarily allows an allergic patient to take the medication by tricking the immune system into accepting it. The procedure most commonly involves initially administering an extremely small amount of a medication (i.e., 1/10,000 of the usual dose). Then, over several hours, gradually increasing doses are given at regular intervals (such as every 15 minutes) until the full dose is reached. To remain in a temporary non-allergic state, the patient must continue to take the medication regularly. After the drug is discontinued, the patient returns to being allergic to it.
Drug desensitization is usually used in patients who have an immediate-type allergy to a medication, meaning symptoms appear within a few hours of a drug being taken. One example of such a reaction is anaphylaxis. Drug desensitization is less commonly used for delayed onset drug reactions, which appear several days after a drug is started. Importantly, drug desensitization is only indicated in situations where there are no acceptable alternative drugs that can be substituted. For instance, penicillin is the treatment of choice for syphilis during pregnancy, as no proven alternatives to penicillin are available. In that case if penicillin skin testing confirms the patient to be allergic to penicillin, then penicillin desensitization is used to safely administer this antibiotic.
Similarly, platins are first-line chemotherapeutic drugs for treatment of ovarian cancer. If a patient becomes allergic to platins, rather than use a different class of chemotherapeutic which may be less effective, desensitization with platins can be performed. There are certain rare drug allergic reactions which are not amenable to desensitization, such as Stevens-Johnson syndrome and toxic epidermal necrolysis. In those situations, the culprit drug needs to be avoided indefinitely.
Learn more about drug allergy symptoms, diagnosis, treatment and management.
This article has been reviewed by Andrew Moore, MD, FAAAAI