Yep, Lion has it covered. The only thing I can add is regarding insurance coverage. Yes, it is true that it will only be covered as a "medical food" if you get the DS (prescription only) form. It may or may not be on your plan's formulary. If it isn't, you may still get covered as long as your doctor orders it with exceptions checked to allow for non-formulary coverage. But, with some plans, there are no exceptions and if it is not on the formulary, then no dice, no matter how many hoops you and your doctor jump through.
The problem is that the research is slim, so there is not enough data to prove efficacy over other treatments. But, so far, there is more research on this than others. Overall, the jury is still out. However, if you are intolerant to antibiotics or they have stopped working, then your options are very limited.
Jan