How to tell if i had an allergic reaction to antihistimine?

This is uncommon: Antihistamines are used to treat allergic reactions. Unfortunately, some people have adverse reactions to antihistamines, but these reactions are not true allergic reactions. They can include drowsiness, dry mouth, GI upset, etc. Thankfully, there are many antihistamines to choose from so their are options. Sometimes, it may not be the antihistamine itself but another ingredient like a dye.