In summary, the term adverse drug reaction is used to designate any type of undesirable and unintended response to a drug and can be broadly classified on the basis of either the presence or absence of an immune mechanism. Allergic reactions (immune) constitute only 5% to 10% of adverse drug reactions. Drug intolerance (nonimmune) constitutes the rest of these reactions. Many of these latter reactions are mild and self-limited, and many drug intolerances cannot be exactly characterized. Of those reactions in which an immune mechanism has been indicated or reactions that clinically appear to be "allergiclike," a limited number of in vivo (eg, skin tests) or in vitro (eg, RAST, IgE-ELISA, other antibody, or cell-mediated assays) tests have proved helpful in the diagnosis. Best studied are adverse reactions to aspirin, penicillin, insulin, and RCM. The principal treatment of all adverse drug reactions is to avoid the drug that has been specifically identified as being responsible for the previous reaction. In cases where avoidance is not possible, desensitization is an alternative (eg, penicillin and insulin). Prophylactic treatment of patients who had previously demonstrated a drug intolerance reaction (eg, systemic RCM reaction) with medication--particularly type I activation--may be helpful in some patients.