My sister takes theraflu warming everynight to sleep, why so addictive?

its not addictive but she has conditioned (taught) her body to require an antihistamine (these inhibit histamine -which is a stimulating neurotransmitter/hormone- which relaxes the body and makes a person tired) in order to sleep well. i think she should switch to benedryl or some type of tablet with less medication in them (this has tylenol, antihistamine, and decongestant) or just try and take less and less so she doesnt need it inorder to sleep.