I've only just started anti depressants- citalopram, but am still feeling very low. how long before they start working?

1-2 weeks minimum: Mechanism of action not known but people often say there's a minimum of 1-2 weeks before they notice any possible benefit (unpleasant side effects usually occur within 1hr to 1week). Some prescribers try to "boost" antidepressant action by adding another medication but it's often better to wait 2 weeks to see if it works. If nothing, consult your provider.