I was on citalopram prior to my new medication. You may experience some side effects in the first couple of weeks (I found they made me very very tired the first few days, as well as a bit disorientated) but all in all, it wasn't as bad as the little leaflet in the box makes out.

The key is to keep taking them... generally, anti depressants take about 2 weeks to kick in for the effect that is intended. Make a note of any side effects you experience, and speak to your dr if you're concerned... but don't just stop taking them unless your Dr says it's advisable