Antihistamines are used to prevent allergies. They work by filling histamine receptor holes before the histamines do. Medications come in pills, nasal spray and liquids in over-the-counter form.
There are two types of antihistamines: first-generation and second-generation.
WebMD states that first-generation antihistamines can cause drowsiness and include Claritin, Benadryl and Dimetapp. Second-generation, such as Allegra and Clarinex, can cause dry mouth.
Second-generation antihistamines should be avoided if you are performing in the near future, but the dry mouth can easily be rectified with hot tea (caffeine-free) laced with honey and a few extra warmups from chest voice to middle to sotto voce.
Voice coach Susan Montgomery strongly suggests taking antihistamines continually during allergy season because there will be less time and energy lost because of runny noses, coughs and itchy eyes. First-generation antihistamines can leave you with ample practice and performance time unhindered.