Older persons should be careful when taking antihistamines. Doxylamine succinate (often branded Unisom) and diphenhydramine (often branded Benadryl) are first-generation antihistamines. These antihistamines are anticholinergics that downregulate the neurotransmitter, acetylcholine. These antihistamines cross the blood-brain barrier and are contraindicated for older patients, such as myself, due to their tendency to impair cognition.

Second generation antihistamines such as cetirizine (often branded Zyrtec) and fexofenadine (often branded Allegra) are less able to cross the blood brain barrier and are thus safer for us older patients, tho, for the same reason, they produce less drowsiness in most people.