The Astonishing Behavior of Twin Molecules

These clips from my Celexa/Lexapro and Provigil/Nuvigil presentations recap the concept of chirality in chemistry: that some molecules, enantiomers, exist in 2 forms, right-handed and left-handed versions, mirror images that seem identical, except that they are non-superimposable in three-dimensional space, and therefore they interact with nature in distinct and astonishing ways.

In the case of citalopram, Celexa is the racemic mixture of both molecules, while Lexapro, S-citalopram (escitalopram), is purely the left-handed molecule, and the only one that actually binds to the serotonin transporter to effectively block the reuptake of serotonin; R-citalopram is inert and actually dilutes the potency of the mixture.

With modafinil, Provigil is the racemic mixture and Nuvigil is R-modafinil (armodafinil). There is no pharmacodynamic difference in this case; rather, the pair differ in their respective pharmaco_kinetics,_ which has to do strictly with how the drug is metabolized and excreted by the body. Armodafinil can usually be dosed once-daily due to its longer elimination half-life.