A Alpha Lipoic acid molecule can have two orientations or structures, called the S and R "entianomers." If you buy a generic (like Wal Mart's REXALL brand), you will likely get a mixture of 50/50 of each S and R orientations (called a racemic mixture). Sometimes, it is proven that one orientation of a pharmaceutical molecule is more effective in treating whatever, and the other half is either doing little to nothing or creates some or all of the negative side effects if any. Take the anti-depressant Lexapro for example. Lexapro is composed of only a certain orientation of the molecule that made up Celexa, that did all the work. A phamaceutical company figures out a way to remove the twin molecule that is not doing the work, and not only gets a renewed patent, but maybe a med that works better with fewer side effects and lower dosages. They can also price it much higher. Sometimes, where the mirror molecule does no harm, it is less expensive to buy the racemic mixture at twice the dose.
In the case of ALA, it should be noted that the liver produces this chemical too on its own, ostensibly adequate for all the body's normal internal needs. Small doses are likely to have nothing but a placebo effect, and is why the studies out there used 600 mg or more per dose. I've seen several studies where a lower dose did nothing at all, probably because the liver is way out-producing what the supplement adds.
ALA is a potent anti-oxidant, and also works to boost Vitamin C and E via regeneration. I take mine, but with food, as I found without food, I can get wicked heartburn from it.