[OMFG!You'reActuallySmart!]

Citalopram is a SSRI and alcohol also affects this system so you end up getting even more serotonin than necessary so you get a double hit of that. Both of them are also "downers" so you get an extra hit of that. In turn, you intoxicate yourself and any effects of drinking alcohol get amplified. I think the biggest risk is it'd be much easier to knock yourself out or do incredibly stupid stuff without remembering it, either leading to nothing severe, something severe to someone else, something severe to you or death (depending on the amount of alcohol and what you do). If you want a fancier answer, alcohol affects CYP450 2E1 (as does paracetamol) and SSRI's would affect other CYP450's, either 2E1 or a similar one. The CYP450's are a group of enzymes that can be affected and do affect the substances (legal and illegal) that you ingest. There's other groups but CYP450's are the big ones.

Sometimes using SSRI's can help someone battle alcoholism because of the increased intolerance to the combination. It's kind of like a cheap version of disulfuram with alcohol, although with fewer of the nasty side-effects if you don't keep drinking a lot or drink only on occasion.