What causes the interaction between antipsychotic drugs and antihistamines? thanks.

Depends: on the antipsychotic drug used, generally speaking drug-drug interaction usually occurs with antipsychotics, antibiotics and anticoagulants (blood thinners), so caution is advised with the use of multiple drugs any how, as far as I know there is no significant interaction between benadryl (diphenhydramine) and zoloft for example, but not sure what medicine you're referring to, kindly run it by your physician