By definition, an allergic reaction to a medication is caused by some kind of interaction between the immune system and the drug. In general, the molecules of many drugs by themselves are too small to engender an allergic reaction. When combined with proteins in the tissues or bloodstream, however, the molecules of these drugs or their metabolic breakdown products become large enough to provoke an allergy.

Evidence suggests that in some cases there may be a family predisposition for the development of drug allergies. For example, there is ordinarily a chance of about one in five thousand of experiencing a severe reaction to sulfonamides or anticonvulsants. But if another family member has had a reaction to one of these drugs, the risk jumps to one in four. For this reason it is especially important to inform physicians about any family history of drug allergies.

As a rule, a true drug allergy does not begin with the first exposure to a new drug. It usually takes between seven and twenty-one days for the drug-protein complexes in the body to provoke the hypersensitivity. The interval between the first exposure to a particular drug and the onset of allergy symptoms is known as the period of sensitization. During this critical period the drug-protein complexes interact with the immune system to stimulate antibodies and a variety of immune system cells. Unfortunately, the frequency of drug allergies increases with age, so we can all expect to develop an allergy to two or three different drugs as we grow older.

The phenomenon of cross-reactivity is another important aspect of drug allergies. What this means is that by developing an allergy to one medication, you may become automatically allergic to others to which that one is chemically related. For example, we know that a great deal of cross-reactivity exists between the various anticonvulsant drugs that are currently available. In fact, about 75 percent of individuals allergic to one member of this group will have a reaction to all of them, and the other 25 percent will have a reaction to at least one other member of the group. In a similar way, persons allergic to penicillin or to one of its derivatives are very likely to be allergic to all penicillin derivatives.

Once you have become sensitized (that is, you have developed an allergy), you usually have the allergy symptoms each time you take the drug. And, frequently, each successive episode is worse than the previous one. In severe cases allergy symptoms occur within hours of the first dose.