Escitalopram oxalate (S-citalopram, Lexapro), a selective serotonin re-uptake inhibitor antidepressant which is the S-enantiomer of citalopram, is in clinical development worldwide for the treatment of depression and anxiety disorders. Preclinical studies demonstrate that the therapeutic activity of citalopram resides in the S-isomer and that escitalopram binds with high affinity to the human serotonin transporter. Conversely, R-citalopram is approximately 30-fold less potent than escitalopram at this transporter. Escitalopram has linear pharmacokinetics, so that plasma levels increase proportionately and predictably with increased doses and its half-life of 27 - 32 h is consistent with once-daily dosing. In addition, escitalopram has negligible effects on cytochrome P450 drug-metabolising enzymes in vitro, suggesting a low potential for drug-drug interactions. The efficacy of escitalopram in patients with major depressive disorder has been demonstrated in multiple short-term, placebo-controlled clinical trials, three of which included citalopram as an active control, as well as in a 36-week study evaluating efficacy in the prevention of depression relapse. In these studies, escitalopram was shown to have robust efficacy in the treatment of depression and associated symptoms of anxiety relative to placebo. Efficacy has also been shown in treating generalised anxiety disorder, panic disorder and social anxiety disorder. Results also suggest that, at comparable doses, escitalopram demonstrates clinically relevant and statistically significant superiority to placebo treatment earlier than citalopram. Analysis of the safety database shows a low rate of discontinuation due to adverse events, and there was no statistically significant difference between escitalopram 10 mg/day and placebo in the proportion of patients who discontinued treatment early because of adverse events. The most common adverse events associated with escitalopram which occurred at a rate greater than placebo include nausea, insomnia, ejaculation disorder, diarrhoea, dry mouth and somnolence. Only nausea occurred in > 10% of escitalopram-treated patients.