Escitalopram is the active (S)-enantiomer of citalopram, which has been shown to be twice as potent as citalopram with respect to serotonin inhibition (Aronson and Delgado 2004). With oral administration of 10 mg, it is rapidly and almost entirely absorbed, and achieves peak plasma concentrations of 10–30 ng/mL in approximately 2–6 hours (Table 47–3) (Rao 2007). Its affinity for dopamine and norepinephrine receptors is negligible. Like citalopram, escitalopram inhibits serotonin reuptake. Radioligand single-photon emission computed tomography (SPECT) studies with patients receiving escitalopram or citalopram indicate that serotonin transporter occupancy in the midbrain was higher with escitalopram (N. Klein et al. 2007).