What is the difference between citalopram and escitalopram?

Composition

Citalopram is a racemic mixture, (i.e., a mixture of the S-enantiomer and the R-enantiomer in equal proportions), whereas escitalopram is the purified S-enantiomer.

Chemistry

Citalopram is a hydrobromide salt, whereas escitalopram is an oxalate salt.

Pharmacodynamics

Both citalopram and escitalopram are selective serotonin reuptake inhibitors (SSRIs). However, studies have shown that the S-enantiomer of citalopram is the active enantiomer, whilst the R-enantiomer virtually lacks 5-HT-reuptake blocking activity. Newer pharmacological studies has documented that R-citalopram actually counteract the effect of S-citalopram (ref.1) and that escitalopram, in contrast to citalopram, has a unique serotonin dual-action (binds to both the primary site and the allosteric site at the serotonin transporter), (ref.2).

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References:

1. Sánchez et al, 2003b

2. Sánchez, 2006