An antihistamine before allergy shots may help prevent adverse reactions. Allergy shots is the only potential disease modifying treatment for allergic diseases (oral immunotherapy is also as well). The typical process of allergy immunotherapy involves regular administration of increasing doses of sensitizing allergy to a targeted maintenance dose. Allergy immunotherapy can come in different routes, subcutaneous immunotherapy (SCIT), sublingual immunotherapy (SLIT), and oral immunotherapy (OIT).
Adverse events can be local or systemic. Local reactions can include redness, itching at the injection site. Systemic reactions can include asthma, hives and anaphylaxis. In general, most adverse reactions happen within 30 minutes after allergen administration.
Antihistamines before allergy shots might serve as an alternative to improve the safety of them. Premedication is usually recommended 1 to 2 hours before administration. Antihistamines before allergy shots reduces the frequency and severity of local and systemic cutaneous reactions, despite not eliminating the risk of other systemic reactions.
A recent meta-analysis study involving the premedication with antihistamines published in the Annals of Allergy, Asthma and Immunology titled “Antihistamine premedication improves safety and efficacy of allergen immunotherapy” looked at whether these agents might contribute to improvements in safety and efficacy of allergy shots.
The results showed that an antihistamine before allergy shots can markedly improve safety and efficacy of allergy shots by reducing frequency and severity of allergic reactions and increasing the target maintenance dose.
Anaphylaxis rates have been estimated to be 1 per 33,000 injections in allergy shots. Premedication with antihistamines may control the episodes and severity of systemic allergic reactions. Antihistamine premedication may also improved the efficacy of allergy shots leading to an increased maintenance dose.
There is also some concern taking an antihistamine before allergy shots might mask minor reactions which could alert signs for subsequent severe allergic reactions. But to date no study has shown a masking effect of antihistamine premedication. In fact antihistamine premedication reduced moderate to severe allergic reactions and anaphylaxis.