Would a prescription antihistamine interact with 20mg celexa (citalopram)?

Usually not: Most are ok to take and no interaction but depends on which med.

Unlikely: Celexa (citalopram) can interact with a number of medications, both prescription and over the counter, but antihistamines are not known to interact. You should avoid decongestants, "nsaids" - aspirin, ibuprofen, naproxen, etc., cough medicines with dm (dextromethorphan), narcotics, and many street drugs.