What is the highest dosage of antihistamine that one person can take at one time?

Antihistamines. Safety of high dose medications is often unknown. Our experience comes from case reports of persons who intentionally or accidentally consumed unusually excessive quantities. When Claritin (loratadine) was first investigated, safety studies were done at 40 mg (4 times the current recommended dose). We take 10 mg because effectiveness is not significantly different between 10 mg and 40mg.
Antihistamine. Follow the instructions on the bottle unless your physician has directed you otherwise. Depending on the illness, antihistamines can be used at higher doses but only under a doctors recommendation.