I recently read in an AARP magazine that antihistamines such as diphenhydramine and chlorpheniramine block acetylcholine, a brain chemical crucial to memory, learning, and concentration. Is this also true of loratadine and desloratadine? Are any of these safe to take in an as needed, occasional quantity or is even one pill bad?   Are there any natural antihistamines that could be used as substitutes?