First-generation antihistamines are commonly used over-the-counter drugs for the management of allergic conditions. However, they commonly have sedative effects and, as such, second-generation antihistamines (H1 antagonists) are increasingly more frequently used.
Antihistamines are often the first drugs used to prevent or treat allergic reactions. In allergic rhinitis, the H1 antagonists are used after the administration of intranasal steroids. In urticaria, histamines are responsible for mediating the allergic response. For that reason, antihistamines are the drug of choice. In urticaria, antihistamines tend to be more effective if they are administered before exposure to the allergen (Katzung, 2015).
In asthma, there are multiple mediators of allergic reactions; therefore, antihistamines are mostly ineffective (Katzung, 2015). In patients with atopic dermatitis, antihistamines are usually administered for their sedative effects to reduce the patient’s awareness of itching.
In some cases, such as anaphylactoid reactions to intravenous contrast agents, pretreatment with antihistamines (diphenhydramine) has been shown to prevent or ameliorate the reaction (Katzung, 2015).