The chemical structure of the drug-some drugs cause allergic reactions very rarely while other compounds (obviously not used as drugs) can cause sensitization in all exposed subjects. Therefore the drug structure is the major determinant of the frequency of an allergic reaction to a drug. Even if in some rare occurrence the dose can be a factor predisposing to allergy (i.e. penicillin-induced hemolytic anemia requires high and sustained drug concentrations) the occurrence and the seriousness of an allergic reaction is not dose-dependent, in most cases. The bioavailability refer to the fraction of the drug reaching the general circulation. Since allergy is not related to dose, bioavailability is not an important factor for an allergic drug reaction. Moreover the oral route carried the lowest risk of allergic reaction.
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