Food additives are ingredients that are intentionally added to food in order to modify its physical, chemical, biological, or sensorial characteristics. Food additives may act as allergens and generate IgE-mediated immune reactions, or they may act as pseudo allergens and generate non-IgE-mediated immune reactions. Such reactions can generate diverse clinical pictures or they may exacerbate diseases as diverse as eosinophilic esophagitis, bronchial asthma, atopic dermatitis, contact dermatitis, chronic urticaria, or anaphylaxis. Thousands of food additives are being used daily in industrialized countries and, although their use has been globalized, there is not much science information about their adverse effects; especially about their hypersensitivity reactions that, despite being reported in literature as rare, it is probably because they are under-diagnosed due to the fact that they require a high level of clinical suspicion by the physician, and the proof of a causal connection between the symptomatology and the food additive. Hypersensitivity reactions to food additives must be suspected in patients who report symptoms with the intake of multiple commercially prepared foods or to a commercially prepared food without presenting symptoms with the intake of the natural or homemade version of the same food or with the presence of idiopathic reactions. The diagnostic role of the in vivo or in vitro test against IgE (Skin Tests or RAST) is limited to some natural food additives. The gold standard diagnosis that shows causality between the additive and the symptoms is the oral food challenge. The treatment shall always be the elimination of the food additive from the patient's diet.