Generic prilosec is a drug called omeprazole that is often taken for acid reflux. Omeprazole is sold as a racemic mixture. While this would matter for many drugs (one enantiomer might be active, the other inactive [in rare cases the different enantiomers can have different effects with disastrous consequences; see: thalidomide]), for omeprazole it doesn’t matter, since as soon as you put it into an acidic enviroment (the stomach!) the stereocenter (a sulfoxide) is protonated, some rearrangements occur, and presto, both the R and S enantiomers are converted into precisely the same bio-active drug.
Now, back in the day when AstraZenica was deriving a significant amount of revenue from Prilosec (prior to generics and OTC status) one of their chemists realized this. So one of their business folk decided, “Hey, when we lose those protections, we’ll just re-submit the drug to the FDA as a specific enantiomer (the S in the case of Nexium [hence the name esomeprazole) and get a “new” drug, and then we can keep charging $200 for a 30 day supply!”
Then one of their sales reps probably pointed out, “Hey, we’ll have to get some studies to show that Nexium is better than the much cheaper Prilosec, otherwise doctors will never prescribe it.”
So one of the medicinal chemists said, “Hey, this drug is totally safe, so we’ll just double the dosage [compared to the 10 and 20 mg available strengths for Prilosec] and the studies will show increased efficacy!” (Which is why Nexium comes in 20 and 40 mg doses.)
Thus a new drug (Nexium) was born. Even though taking 20 mg of Nexium is exactly the same (as far as your body is concerned) as Prilosec, you have to pay much more for the former. This way AstraZenica gets to make beaucoup bucks for another 10-15 years and do absolutely no scientific discovery.