Is escitalopram more potent than citalopram?

In vitro, escitalopram is more potent than citalopram, due to the absence of the R-enantiome. In vivo, escitalopram is also more potent than citalopram in different animal models of depression (acute and long-term) and in models of 5-HT uptake inhibition (ref.1).

Potency can mean that escitalopram has a better antidepressant effect per mg than citalopram (i.e., it works better because it is stronger). However, it may also mean that it is more efficacious.

Animal studies (using the microdialysis technique) show that escitalopram produces a larger increase in serotonergic output than citalopram does. These studies show that R-citalopram counteracts the 5-HT enhancing properties of escitalopram. Escitalopram's serotonin dual-action in binding to two sites on the serotonin transporter (both the primary site and the allosteric site) is hypothesised to be responsible for a larger binding to, and therefore greater inhibition of the serotonin transporter (ref.2).

References:

1. Montgomery S et al. 2001

2. Sánchez C. 2006