Citalopram

Citalopram works by blocking the reabsorption of serotonin, a brain chemical linked to mood, so more of it stays available between nerve cells. The pill people take is actually a 50/50 mix of two mirror-image versions of the same molecule, but only one of those two versions — the 'S' form — does almost all of the actual work; the other version barely helps and may even get slightly in the way of the S-form doing its job. That S-form was later purified out on its own and sold as a different drug, escitalopram, also on this site, at a lower dose. Citalopram also carries a specific warning: taking more than 40 mg a day raises the risk of a heart-rhythm change called QT prolongation, so doses are capped there.