As the above poster stated, Citalopram (ie Celexa) would not be tested for on a pre-employment drug screen, as it is not a controlled substance or illicit drug.
Just general info, always the best policy to be 100% honest when you're asked about medications/drugs you've ingested prior to the testing, rather than trying to skirt by and not have something detected. I'm assuming the Celexa was prescribed? If so, it doesn't hurt to mention it to the screeners.
Hope that helps, good luck!
Citalopram shouldn't show up on a drug screen because it's not a narcotic. It's an anti-depression medication. If you are really worried about it call a Pharmacist...they should have that answer.