When should you take an antihistamine?

For allergies: Antihistamines help the most with sneezing, watery drainage, and itching which are often seen with allergies. Older antihistamines (like benadryl) can make people sleepy and should be used with caution; newer antihistamines (like Claritin (loratadine) & allegra) are less likely to be sedating. Because of their drying effect, some doctors recommend avoiding antihistamines during a respiratory infection.

Multiple reasons: Depending on the type of antihistamine used they can be used to treat things such as allergy symptoms, nausea, dizziness, motion sickness, anxiety, itching, and difficulty sleeping.