One purple pill is much like another, perhaps even more than you might think. You can't open a magazine these days without seeing an ad for "the purple pill", Nexium. The original purple pill, Prilosec, was the number one selling prescription drug in 2001. A proton pump inhibitor, it stopped the production of acid in the stomach at a different step than earlier heartburn drugs such as Zantac. Patent protection for Prilosec expired that year, and Nexium was introduced with much fanfare. The FDA approval was based on the argument that Nexium was more effective than Prilosec. Now for the chemistry trivia: Nexium is exactly the same molecule as Prilosec.
To understand the relationship between Nexium and Prilosec, you need to know that many organic molecules come in mirror-pairs. The molecules have exactly the same structure, but are mirror images of each other. Your hands are a good example of this kind of mirror pairing, and in fact the chemical term describing this phenomenon, chirality, comes from the Greek for hand.
Many biological systems show a preference for one mirror image over the other. For example, all the proteins in your body are made up of amino acids which exist in these mirror pairs. Naturally occurring proteins all use only one of the two mirror images, called the L form. Perhaps the most striking example is that of spearmint and caraway oils. Rye bread and spearmint gum taste very different, but the molecules responsible for the taste are identical except that they are mirror images of one another.
Back to Nexium and Prilosec. The active compound in Prilosec is omeprazol, which comes in two forms, R-omeprazol and S-omeprazol. The R and the S come from the Latin for right and left - just like the hands - and indicate that this is a pair of identical mirror images. Both R-omeprazol and S-omeprazol are effective in reducing heartburn, but S-omeprazol is about 4 times as effective as the R form. Prilosec contains a 50/50 mixture of the R and S forms, Nexium is pure S. So 20 mg of Prilosec is equivalent to 12.5 mg of Nexium (10 mg of S + 10 mg of R/4). The prescribing literature on Nexium obscures this fact, giving a new name to S-omeprazol: esomeprazol. Get it? "es"omeprazole.....
If you want to learn more about chirality, check out Prof. Jean-Claude Bradley's organic chemistry lecture. Lecture 16 introduces chirality. The lecture is available as a podcast or streaming video.